Welcome to the site.  I would say that it really depends.... and I'm one of those people that came out the other side with adult steps that I have a good/decent relationship with... but I did not have as many of the issues that you may be experiencing.

Only you can really make the full and balanced decision.. because you life the full extent of it all.

1.  From most posts here, you will see that things generally don't get better.  They may become different.. but the problems you seem to have going on are not likely to resolve or improve.

   First, you have a kid that while he may have some ADHD.. that doesn't encompass all the other things going on.. resentment.. mistrust etc..  are those going both ways?  do you mistrust each other?  Do you try to discipline her child? (do not do that.. btw.. bad outcome will result)

   Second, you have an EX who apparently actively undermines you, the child will have loyalty binds.. not allow themselves to llike you etc..

  Third, you have a mom that apparently is not addressing the jealousy.. the resentment etc.. she is not setting appropriate standards of behavior for her son in her home.. that is a HER problem.. she may love her kid.. but she may not be an effective parent and may not be supporting you as a partner either.

2.  A lot of the issues with the first part.. help with the jealousy issues.. if you are able to get to a point of peacefully coexisting without her son feeling he is being pushed aside or displaced.. that's how it works.. in your case.. I'm doubtful that will happen though.

3.  I wish I understood that just because I'm a sane and rational person that everyone else will be as well.  I didn't try to overstep but the EX was extremely jealous and unhinged at times over "her being the mother.. not me".. etc.. You also have little control over what the EX says and does.. all that can be addressed is what happens in your home.

4.  For the step parent.. I would say that someone who is well suited will be able to have the capacity to see other perspectives.. empathize with the fact that everyone has some issues to overcome.. it's not easy for a child to accept an authority figure that is not their parent.  It's not easy for the EX to accept that they can't control who is in their child's life.. it's not easy for the partner/bio parent to balance everyone's needs.. the child is their number one obligation.. but everyone should feel that they are a priority..   Being flexible.. not feeling a strong need to lay down the law for rules.. not overstepping and trying to force our own personal thoughts on "how a kid should be raised".. at the end of the day.. the kid has two parents.. they should parent.. you can support your partner if they need assistance with rides.. or mowing the lawn.. but discipline.. reminders to do chores. or even IF the kid has chores is a parental choice.    For your partner.. she has to be able to make you a priority.. to not undermine you with her child.. not say things like "Oh.. snookums.. please clear your plate so stepdaddy will not get mad".. She will have reasonable boundaries with her ex.. she will be a full partner financially to you.. she will set boundaries and standards of behavior in the home for her child.. and parent them to act appropriately.. and be civil etc..

just another bit more.. I'm not sure how long you have been in the relationship.. how your relationship with the child started.. any actual conflicts.. any misteps you may have had with the child... what the schedule is like.. do they have a CO.. does he pay CS... what is the financial set up with you.. 

Are you willing to be beholden to her custody agreement for the next 10 plus years?

Go with your gut!!!

Trust your intuition and your womb!

Plenty of women on this forum pregnant or with toddlers very unhappy with their partners inability to prioritize effectively and considering divorce. 
 

My ex fiancé and I discussed marriage and kids when we first met ... then his meddling conniving ex wife and her mini foot soldiers did everything in their power to interfere 

He was a staunch Disneyland Dad operating a 24/7 mobile ping service (aka B*tch Beck n Call LLC) with his exwife as the CFO and his kids as the team leads.
 

My womb helped me realize that if I married him in his current state (I'd be locked in to his failed former family dysfunction) and if I had a baby with him (I'd really be locked in plus I'd risk being a married single mother while he neglects his new family to overcompensate for his divorced dad guilt)

I may have been willing to give him the benefit of the doubt and get his act together but my womb and v@gina was very selective and would override any fantasy that was not reality in which me and my future unborn child could proper without interference from some disgruntled meddling ex and maladapted stepkids unnecessarily taking lion share of our household resources. 
 

Your womb knows the answer and the fact you had to even ask the question means deep, deep, deep down you know the answer to your own questions 

*my bad I think op is a man but you know what I mean though

1. Does it get better? What are examples? What helped improve the situation?

Yes if both parties are willing to improve 

Therapy is one solution 

2. How often does an already jealous step child grow to accept a new baby?

As often as the bio parent who instills proper home-training and fosters an environment of 'love your siblings' (not hierarchies/favoritism)

3. What do you wish you had known before stepping into a step-parent role?

How to say NO! ...Final answer ... and not feel one ounce of guilt or regret 

4. What kind of person is well-suited to handle the challenges of blended families?
 

Someone who is used to large families, a little chaos at times, good with kids, patient, and married to a partner who compensates very well for the effort dealing with the nuisances of blended families 

Crank up the jealousy to new levels.  Expecally the weeks SS is with his BF.  Your baby will be getting all these fabulous things. Going on fabulous trips to the food store.  Ect.  Not going to be good as things are going on.   
'WHY didn't SS get a full mental evaluation and work up ?  To know what's actually really wrong with him?   That would be a good first step.

When SS is away for his BF week, bio baby was given 4 ice cream cones. You now own SS 4 ice cream cones. This will be the crap you will be getting. SS missing those fabulous weeks of alone time with bio baby.  Leave find a uhdamage SO. 

No, it doesn't get better. You don't have an SS problem, you have a SO problem. She's even admitted to being too lax in her parenting to make up for her ex' abuse of SS. The crux of the matter lies within your SO, rather than your SS. 

When my DH and I had our DD24, it was an unexpected pregnancy. I didn't want children because my SKs 32 and 34 were literally feral and my DH turned a blind eye to everything they did. BM and DH infantilized them so they were very weird kids and the odd times that BM or DH would decide to try to parent the ferals, the SKs would fight back and BM and DH would throw their hands up. Then, when DD was born, both DH and BM panicked over the little darlings' fee-fees being hurt, so they ramped up the coddling, which was already off the charts. Over the years I would have one foot out the door and DH would go to a counselling session or two and then at any sign of evolvement/improvement in the SKs, especially the mini-wife on steroids, DH would sabotage everything and knock everything back to where it was. He felt he just "couldn't afford to heal," or he'd lose the adult baybees, especially SD. Today, SD is stb35, a BM herself, and causes so much strife even though she finally moved out at age 27. To this day, not a day goes by that I don't regret not leaving YEARS ago. SKs who aren't parented properly do not meet milestones like their peers do and they do not leave home. I was exhausted trying to make sure my DD turned out completely differently from my SKs. I could go on and on and on. Run fast and run far. Things won't get better. Your SS will get bigger and he'll get worse. I've seen stories on here where SPs have been assaulted by their unparented teen and adult SKs. I've seen many stories on here where SPs are stuck because they're scared to death that their young child will have to go to ther other parent's house half time with an out of control, unsafe brat. Don't do it. End it now. For the love of God wrap it up and don't get your SO pregnant. 

Ask yourself this one question: what's changed?  Because unless your gf is actively working the problem (consulting her son's pediatrician and behavioral specialists to craft a treatment plan; counseling for him; parenting classes for her), things will only get worse as the boy gets older. Teen boys WITHOUT ADHD are already challenging, with mood swings, anger etc - imagine this poorly parented one feeling his oats.

Evil4 has laid the future out for you. You need to get OUT. Heck, your gf should be asking you to move out so she can fully focus on getting help for her son. That she hasn't is quite telling. Date her while living apart if you must, but make it clear you're giving her the space she needs to get help for her son without you being a distraction.

No, having kids didn't make things any better and actually made things worse.  SD acted out and as she got older and became promiscuous she started sneaking men into our house. One had a history of child endangerment.  We separated for a while and only let DH back in the condition she stayed gone.

it's been several years since that happened and things have improved but for a long time the only reason I came home sometimes was bc my kids were there. 
 

The other thing to keep in mind is that my children witnessed the crap and although my son is OK my daughter won't come here unless she has to when SD is visiting 

Whether or not it gets better will depend on your SO. If she lets the child set the tone and call the shots, it will not get better. As an example, when I met my SSs they were 9. They referred to their stepdad as "our dad" and their stepbrother as "our brother". BM shares photos with "my three sons" and "my heart" (that include her stepson. Fast forward several years and one SS no longer gets alone with Stepdad and BM HATES her stepson and fights with him daily. No suprise, BM and stepdad divorce. SSs bragged to DH that they "broke up mom's marriage." BM has always let SSs call all the shots. They always decided where they would go, if they wanted to do something or not, etc. 

First, welcome. I hope that you find this to be a good place to vent, contribute, and to pick up some useful perspective from others who are living or have lived the blended relationship dream.

It is good to have another man-spective on STalk. Like you, I am a non breeding step dad.  I met my DW of nearly 30 years when my SS was 15mos old. We did not have the complexity of a long history of my DW and the Spermidiot being a couple to overcome. DW kicked him out for  cheating with yet another statutory rape victim before SS turned 1yo.  We also did not have the complexity of a local visitation schedule. DW left SpermLand shortly after she graduated HS with a baby on her hip and never went back other than for visits. We met at University.  We married the week before SS-31 turned 2yo. This is the foundation of my experience and success as a StepDad, DH to a prior breeding mom, and as equity life partners in a life of adventure and a love for the ages. A week from today we will celebrate our 30th anniversary. 6 days later our son turns 32.  

As it turned out, SS is an only in our marriage.  So as to having a child with a prior breeder, I do not have that experience to reference.  Though I would point out that your SO is a failed parent and the odds of her being a better parent with a child she has with you... not likely. Past behavior being the best predictor of future performance makes it exceptionally unlikely that she is capable of being the parent you would want to partner with in raising a child of your own. IMHO of course.

As for success as a SParent and in a blended family marriage. KISS. Keep It Stupid Simple is my philosophy.  The complexity and variables are endless and are little more than a distraction and excuse if the SParent in the mix tolerates the endless excuses, emotions, and distractions.

KISS, IMHO includes some absolute truths. First, you and your mate are equity life partners and that includes being equity parents to any children in your marriage regardless of kid biology.  

Set the standards of behavior and standards of performance you require in your home and enforce those standards in an age appropriate manner for any kids in the mix. Hers, yours, or any you have together.  

Do not overcomplicate it or you will be overwhelmed. Keep in mind that second marriages fail at an even higher rate than initial marriages and having failed family progeny as part of your marriage far more likely than not induces an even higher probability of relationship failure.  I am divorced. My DW is not. Though she is the one who brought a  kid to our marriage.

1. Does it get better? What are examples? What helped improve the situation?

It can be great. But only if you and your FDW are absolutely on the same field regarding parenting. Only if you are crystal clear on YOUR requirements for kid behavior and kid peformance in your home and marriage. Her X is irrelvant, her kid's issues are irrelevant.  The standards are completely relevant and they are to be adhered to. Period. Dot.  Anything else just degrades the quality of life for everyone in your home/marriage.

We have had a great life and made a strong family together. Our son (my SS who asked me to adopt him when he was 22) has always been a key part of that.  Our anniversary is also his anniversary. We started this whole thing together and remain very close.  Raising a man of honor, character, and standing in his life, profession, and community is a point of pride for both is mom and I.  It can happen. But .... KISS is critical, Standards, standards, standards.

Our particular cross to bear was the SpermClan. Most specifically, SpermGrandHag. Manipulative, toxic, PASing.  We did the KISS model with them as well. They behaved, or they suffered to the fullest extent we could bring to bear.  We applied every legal, financial, and social consequence available when they plied their crap.

It took a couple of years but we did land on zero tolerance for pre visitation behavioral degradation or post visitation behavioral detox.  Our standards applied from the second he came home from SpermLand visitation until the moment we put him on the plane to fly to SpermLand.  Zero tolerance was the KISS element regarding visitation related behavioral crap.

2. How often does an already jealous step child grow to accept a new baby?

I have no idea. However, KISS would indicate that the SKid has zero choice in the matter. The standards of behavior and performance apply whether you and your mate add children to the mix or not. Do not give the failed family progeny a choice and .... there is no need to worry about it other than to apply appropriately effective misery inducing consequences if the Skid chooses to make their own life difficult regarding a baby sib.

3. What do you wish you had known before stepping into a step-parent role?

I think that the forcast of what ifs is no less a waste of time than are active what ifs regarding an active Sparent life.  Just set your standards, enforce your standards, and tolerate nothing less than absolute immediate adherence to those standards. KISS.

4. What kind of person is well-suited to handle the challenges of blended families?

Anyone who is confident in who they are, clear on the fact that they and their mate are equity life partners and equity parents to any children in their marriage regardless of who the X made/breeding partner is, clear that any Xs in the mix are irrelevant and will absolutely bekept in their place, and clear that there are inviolable standards of behavior and performance regarding any kids in their marriage, is suited to navigate the challenges of a blended family. The major issue is... your mate. If they are already demonstrating that they are a failed parent, the odds of them becoming anying else even with children they have with a new mate are slim and non at best.  Never delude yourself otherwise. Also never forget that a blended marriage is far more likely than not a second marriage for at least one of the spouses in it.  Be true to you, who you are, keep things as simple as possible, let  your mate know that you have their back while demanding that they have yours, and let it rip.  Good luck.

I don't have children so I know I'm naive as to how kids really are.

Sadly this is typical bullshit spouted by many breeders in the direction of non breeders. IMHO, it is complete delusional manipulative bullshit. Everyone has been a kid. Everyone has had parents. So everyone knows how kids really are.  Just because someone may not have had children of their own does not negate their life long exeriences navigating their own childhood or the example set by their own parents.  Everyone sees how kids are at some point every single day.  We see well behaved outstanding children of outstanding parents. We also see clear breeding mistakes produced by idiot parents. And everywhere in between. So, do not discout yourself as an observer of the parenting outcomes of any kids and parents you see each and every day.  You know a good parent and a good kid when you see one. The converse is also true. Again... KISS.

What I realize is that I cannot expect anyone to change.

This is absolute truth. However, there is no need for anyone to change if the boundaries regarding standards of behavior and standards of performance are clear and enforced.  The only consideration is... are the standards adhered to, or not. KISS.

Standards not only apply to any kids in the mix, they apply withing the marriage, and they apply to any Xs, ILs, etc...   Again. KISS.  Do not allow your standards to be violated without immediate definitive consequences.  For damned sure do not waver on this when it comes to a toxic X. Either yours, or hers.

 I just question whether I want to step into a blended family dynamic or if I am letting my fears sabotage an otherwise great relationship with my partner.

This is where the ice regarding a blended marriage gets very thin.  

otherwise great relationship with my partner.

Other than that Mrs. Lincoln, how was the play? 

Be very aware and wary of polishing a turd when it comes to a mate, a Skid, or a blended family marriage. Far too many SParents waste countless years polishing what is actually little more than a turd. Never forget that even a polished turd is at best just shiney shit. If your mate cannot and will not be an equity partner, parent to YOUR standards as you and they are equity parents to any children in your marriage, and commit to your life together as strongly as you do, believe them when they show you who they really are as a mate and as a parent.  Do not polish a turd. It is a waste of your own life.

Remember, an ill behaved StepSpawn is a failed faily progeny.  The odds of them being anything but what they are are slim and none at best. As kids, and as adults.  Good kids tend to grow to be quality adults. The case of crappy kids is that they will always be what they are. Even as adults.  The critical success factor is your mate.  If your mate is raising a well behaved kid before you join the mix, the odds are mostly forever in your favor. If not..... be very, very, wary.

When it is right, the sky is the limit.  Just know that the higher you may fly the more catastrophic the crash if you choose to ignore the facts of it all.

Take care of  you.

All IMHO and experience of course.

Good luck.

As for the ADHD element and any other pseudo science condition of the moment that may apply, the standards apply regardless IMHO. As it turned out, my SS is ADHD. There were some subtle indicators when he was a child but nothing DXd.  He thanks us fairly regularly for the stability of his childhood and us holding him accountable for his choices and behaviors.  He also appreciates that we set an example of strong confident adulthood  as a mom/woman and as a dad/man and a strong mutually successful marriage.

In hindsight, I like to think that we would not have abandoned the standards of behavior and performance he was raised with even had he had been Dxd with ADHD as a kid rather than an adult.  IMHO in large part his success as an adult is due to the example we set and the standards we required of him.

Take care of yourself in all of this.

As these kids get older.  As in teens they get worst. Hormones changes.  Many mental illnesses don't show until teenage years.  You are going to spend your life devoted to SS. and at 20 he going not be talking to you or just asking for $.  This illness has to be someplace in DW Family or BF fanily. If it from your wife side. There a chance it will be passed on again.  Don't give up on having your own family . Because that's the only family you will have