The issue with your situation is not necessarily the ages of your step kids when you entered it, it's that the situation is dysfunctional, and it was that way before you came along.

How long have they been divorced? How many relationships has your husband had since the divorce?

I'm an adult SD and a SM. It took me a long time as a SM to be okay with the probability that my SSs weren't going to seek me out for advice or help, and I'm in their lives as they are still minors. They have two parents already that they love and find support in, and while I rarely agree with their mother and have differing opinions than my DH, it's not my right or responsibility to step in. It's a tough position to be in as that's not what the message to SPs is (which we're told to love our SKs as our own and treat them as our own). However, reality is that I'm not the parent, and they are under no obligation to seek me out for help, or to form a relationship with me. They need to be civil and respectful for sure, but that's the extent of their requirement. They didn't marry me or ask that I become family, so they get to dictate what our relationship will be outside general parameters of how children should act with adults.

As an SD, I have a SF who has been in my life since I was 15 and my dad has had several GFs over the years. I have a more parental bond with my SF than I do my dad's GF, but my SF has acted in a parental role. I was 28 or 29 when my dad started dating his GF, so I didn't need a parental figure. That's not to say that I don't like my dad's GF, and I need to do a better job of getting to know her and build that relationship (she has offered an invitation for that). But, while she is part of my family, I don't seek her out like I would my mom or dad, or even my SF. She's still new, and I'm building a relationship with her just like I am with my sister's fiancee. But just as I don't consider my sister's fiancee another sister, I don't consider my dad's GF another mom.

Don't try so hard. Offer advice if your DH wants it. Be polite and kind and open to a relationship, if that is what you want. Accept that they may not want a relationship with you that is deeper than surface level. Accept that if they do want a relationship deeper than surface level that it likely won't be a parental relationship. It may not even be one where they want your input or advice. 

My best advice is to accept politeness and cordiality as a win, and stop trying to develop something more. You may never be more than their Dad's wife, and that's ok and better than lots of step situations.

I made the mistake of trying too hard to be accepted as part of their family--time, money, attention. No more. All the effort on my part is meaningless unless there is reciprocation.

Now, I'm tolerated, and both adult steps are glad their dad is happy. That's it. I am sometimes sad that there isn't more of a relationship, but they are adults and can choose to build a relationship with me or not. I'll take cordial over open warfare any day.

“Even when an issue with the SS was concerning DH and myself, he turned to BM . . .”  Well, I bet that made you feel like shiatsu and justifiably so.

I am sorry, BeautifulBird.  I just read this.  But this really is inexcusable from your DH.  He really is, himself, acting like his ex- is his “senior” wife.  Regardless of his rationale, when he went to BM for advice regarding an issue that was supposed to be between you (doesn’t matter if it involved SS or not), he made it clear to her and their children that she was “senior” wife, and you were second.

I get that you are not so experienced and have no kids of your own to judge things by.  I am bioless as well.  BB, your husband is unintentionally? setting you up for failure.  You are not some other family’s lackey.  You are your husband’s wife and deserve to be treated like a wife and not like a second wife.  There really is no such thing as second wife in the US.  We are not a polygamous country.  As long as you allow servant-lackey treatment, believe me, your DH, kids will not change.  You see, they all think they get to have their cake and eat it too, incl. BM, while you are supposed to settle for scraps.

Before you can be more friendly with SS and SD, your DH needs to get it straight himself; otherwise, he will just keep undermining you.  First, he needs to understand himself that you are his one and only wife, and secondly, he needs to keep projecting this to his now adult children.  Fairy tales don’t happen any more with step-families than they do with initial families, even less so.  Any marriage takes a lot of work, and a step-marriage takes even more.

Rather than thinking about everyone being buddy-buddy, first you and your husband need to be on the same page, with both of you knowing that you are his one and only wife vs. just you.

BB, I know I emailed you earlier about how I was treated at my SD’s wedding.  What was so humiliating and disappointing and upsetting about all of that was how my DH’s marriage to me was so poorly thought of by everyone, including, and I’d even say especially, clergy, to the point that they felt they had every right to take me from my husband’s side, without permission from either of us and after fifteen years of marriage between my husband and I and all that I did, and place him right next to his ex-, and neither I nor my DH could do a damn thing about it.  This was my one and only marriage, and no one, no one in that church that I know of, recognized our marriage as being legitimate.  I was completely left to fend for myself.

And, that is despite the fact that DH and his ex-wife had been divorced for years!  And, BM already had another marriage and divorce under her belt, in addition to my husband.  This is how SMs are thought of.  My DH did and said nothing at that wedding to discourage or correct any of this.  I tell you, if there was ever a moment I had an epiphany about how others saw my marriage, that was it.  SD’s marriage was in a Lutheran Church, and they are supposedly more liberal.  Yeah.  Love the way everyone in the US acts like divorce is okie-dokie, but if you marry a divorced man with children (not even remarry; just marry), all you’ll be seen as by most is as sloppy-seconds.  AND, in my case, it was my first marriage.  My first!  Do you know how it feels to have your first marriage thought of as sloppy-seconds by seemingly everyone?  Unfortunately, that is what you are seeing now.  That’s the reality.

So, if you care for your DH and love him, which I’m sure you do, then you’ll fight to get him to see that you are his one and only wife; otherwise, he’ll keep doing what he’s been doing, and you’ll become angry, resentful, and bitter, and 10 or so years from now, you’ll wonder and feel like you wasted 10+ years of your life.  Don’t make excuses for him.  HE needs to see that you are his one and only spouse.  That’s the first step, after research.  Once you accomplish that, then you can try to move on to blending better, if it's reciprocated.  But, without DH as your one-and-only husband, by your side, all the way, there is no way any blending is going to occur.  And, BM?  Put a big fat X through her, because that is what she is, an ex-.

Hi , i'm unable to sleep due to a big fallout with my fiancé's adult daughters. Yesterday was the worst day & i'm now wondering how we will be able to move forward from this. 
i moved in with my partner 4 yrs ago this month & we are now engaged. We both have adult children however he is a widow & both my partners daughters have keys the house they grew up in. They have always let themselfs in and in the past without any warning and even use to walk straight into our bedroom where their father & i would be in bed. I didnt know how to deal with it at first as firstly i was new to this situation & 2nd i didnt know how to deal with it without being rude & you want to get on with the children when your new to a family as its delicate ground plus i also felt for them for losing their Mum to cancer.  however i did tell my partner quite clearly one day after 3 yrs  that i was'nt comfortable with it anymore as one day they just walked in when he wasnt even there and i was half naked out of my bed covers. So after an huge argument a lock was finally put on the door. This was after they moaned about me to their father for asking them to go out my room as i was in bed.
When we got engaged and after he's eldest daughter got married,  i thought they would finally figure it out that as adults we all need some kind of privacy. (Even if they do look at the house as their mothers they surly must be able to understand we all need privacy) . However its just got worse. Quite often they would let themselfs in when we was both in our night wear in the early morning / late morning or turn up in the evening when we was about to get intimate or just dressing after being intimate and a few times i had to beg my partner to leave the key in the door incase they turned up. Luckily on a few occasions it saved us from embarrasement. Quite often aswell they would just turn up when their father wasnt even at home yet i was , saying they wanted to see the dog ( family dog their father still has). I would never have a choice on if i wanted company or not, its always just been pushed on me as they say its their mothers house and their dog. After a few yrs it really started to upset me as i felt they didnt respect the fact that it was now my home where i lived with their father. Not my house but deffinitly my home & planning to be married. We all need our privacy dont we ? Or am i just out of order for needing that? I'm a very private person and my privacy is very important to me, my own children always make plans days in advance before we see one another or we visit each other. I'm not asking them to do that but the day before or the morning of the day with at least a few hours notice would be nice. It gives you time to prepare for company or tidy up or dress nice before they come round.
Last yr their father tried speaking to them asking if they could give notice before turning up as he could be in he's pants. They took total offence to it and there was stress which did clear after a few days but what they did , was they would Let us know when they where sitting on the driveway outside ringing to say they are there,  or 10 minutes away. They would still let themselfs in and sometimes if i wasnt ready if they turned up early in the morning or late in the evening  and i was getting ready etc  i wouldnt come down to say hello to them , as i felt they still hadnt really respected their father when he asked them to give warning.  So i would say bye as they where leaving and politely say sorry i was busy upstairs . Then yesterday was the worst. I wasnt feeling great and we hadnt got ready all day  as we had a lazy day as we both had a hectic work week. Then around 4pm my partner said he would pop to the shop to get dog food etc so i said i would wash my hair so we could have the evening free to enjoy together. He left the house & within 2 minutes rang me to say he's daugher was on her way round to see the dog. I said "look could you ask her to pop round tomorrow its just your not here & i'm not up to company right now i'm not even ready. I feel uncomfortable" He rang her , she totally ignored him and turned up at the house anyway letting herself in. I just could not believe it. Her father even rang me to say she wouldnt take he's calls. I locked myself in my bedroom. I then received measages from her sister totally insulting me as her sister had moaned about her dad saying not to come telling me "i am controlling and its their mothers house , there dog and i am being unfair". I was  so upset at the sheer shock of disrepect towards not only their father but towards me aswell that my hands where shaking so much i couldnt even type a message on my phone to my partner to say how upset i was. I also knew if i came down from my bedroom and spoke to the one step daugher i would of been so upset, i dont know what i would of said. Thats why i stayed in my room. The messages from the other step daugher where so insulting even telling me how they had moaned about me to so many of their friends and  how controlling i am for not letting them come round whenever they want. In the end i thought ' i do not have to keep taking this'. So i messaged along message to say how "both step daughters had their own homes with their partners, how even their own father makes plans before he would ever just turn up at their homes and how even though its not my house it is my home  and we all need privacy sometimes and i will not put up with the way i was being spoken to" . Then in response was told " how they never wanted to speak to me again and they was deleting my number. " when my partner came home he was in a state as they had been moaning at him and so  we argued and i said " you have to do something about this , how can we get married when i have no say in anything and you promised me we are going to move to our own place one day you keep saying it, as this is never going to stop . They can never have  keys to our new place as they clearly have no respect towards me or even you as they have shown us that today. This is my home where i moved in to be with you and even they have told us sometimes that they dont always want company. So why am i not allowed or you allowed to say that to them and one of them is now married. As their father surly you know this is not right?" 

we argued for ages after but he did  tell them they should give him 24 hours notice before they come. He said he is in the middle of all this and he wants everyone to get on. 
i feel like after today they have told me exactly what they think of me and shown me they are not properly willing to except i am not only a human being but am engaged to their father and deserve decency and i know in myself this is going to be so awkward now. It will probably never be alright now. Dose anyone have any advice ?  I have read this post page last yr about trouble with step daughters but finally plucked up the courage to post on here. 

After 35 years of trying, trying, and trying, I can tell you I wasted my time.  We have been married 40 years.  I learned the hard way.  I have walked away from the step mother job.

The facts are: You didn't raise them.  You cannot change their habits.  They are adults.  You will always have stepchildren, as long as you are married to your DH.

Let the past be.  Ignore their dysfunctional life, you can't change it.  They are adults, be civil, not best friends.  All of this I do now, after years of self induced mistreatment.

Take care of you, your marriage, and don't let them use you.

Have a fun life without adding stress.  

He is not in the middle of it.  This IS NOT a daughter vs. SM deal, where poor DH has to choose or be put in the middle.  He needs to treat his wife like a wife and his daughter like a daughter.  Even in initial families, daughters don’t get to just barge into mom and dad’s room, and even in many initial families, adult children are not permitted to just come over as they please. 

These are your DH’s children, so it is his responsibility to make sure that they are clear and understand this.  Sure, maybe they have gotten away with this in the past, while DH was still single.  He let his daughters come and go as they please, and apparently, even into his bedroom.  Now he has a SO and soon to be wife; thus, as a married couple, the marital home belongs to the married couple and rules are not made and set by children, whether they are adults or not, or whether they were permitted to run feral throughout the home in the past or not.

And, you are correct, you’d think any adult would get this, but they see themselves as still owning their father (by being a pansy, he is feeding into this by the way), and you as the outsider who is raining on their family-in-the-past parade.  It is dad’s role (not yours) to make certain that his own children see you as his partner, his spouse.  Again, he needs to treat his wife like a wife and his daughters like daughters. 

Personally, I’d insist that the house, rooms in it, be rekeyed, and then dad can go over and just tell his daughters that as any married couple, you deserve your privacy, and that anytime they want to come over, rather than using their old keys, they’ll just have to call.  I mean, really!  If he won’t do it, I’d do it myself.  Either that, or you and your DH can just come on over to their home any time, barge in, walk into their bedroom, sit down and start eating their cereal, etc.  What is it they are not getting?  It is about control and them not wanting to give any of it up, unfortunately. 

You’re his wife, and if you’re not sure, ask your DH or put it to him that way:  Who is more-so your wife?  Me?  Your daughters?  Sounds creepy and weird, but you’d be surprised how many DHs seem to conveniently “forget” whom they are currently married to, especially when it makes things easier for them.

The girls are insisting that things with him have to be like the good ol’ days.  They need to realize that he is married (or soon to be) and is moving on and creating a good, happy life for himself, and that means some things that were let go in the past (and probably never should have been), are going to be addressed.  AND, it is up to your DH to address them.  You are not responsible for reigning in his adult children.  That is his job.  But if he won't budge or rekey the house, you do it!.  And leave it to him to explain.

Since the kids seem to think it is their home and their father's home and the dog's home and their deceased mother's home, BUT not yours. . . .  I think a good suggestion, and not just in this case but really when blending any family, is to eliminate the old home (sell it), and get a home together--your own home.

If your DH states something to you like, "Oh, I can't do that to the girls," then you have your answer about his priorities.  It seems like their mother passed years ago, and it seems they can get married and move on from the 3-some, but apparently they are hell bent on not allowing their father to.  He needs to recognize this, and recognize that he is not being put in the middle.  What he is is allowing himself to be manipulated.

Sell the home, get a new one, and do not, under any circumstances give these girls of his a spare key.  If they want to see the dog, they can take it for a day or week or so here and there.  You cannot use a dog as an excuse to treat human beings like shiatsu and deny your father his right to his and his wife's privavy.  They should know better, and if they don't, your DH needs to force their hand.  

He can sell the home, his girls can go through and take what momentos they want, and the next home is an "ours" home, you and him, period.  Girls can come when invited or if they ask first, also period.