I have this situation, too, but the SD is 58, not 20.  I could go on endlessly but if you have read on here about mini-wives, you know the story.  I wish I could tell you how to stop it, if you find out, please post.  It really gets me with that "sensitive" bit. If I say something resembling reality to her, he gets worried because she is so terribly sensitive and anxious.  So everything is supposed to be sunshine and rainbows because of her "sensitivity".  Barf.

One thing I've figured out: they both get something out of it.  She gets attention, money, closeness.  He gets the feeling of being a good dad, appreciation, being needed.

I have to say that when SD married each time and was busy with her kids, there was less of this behavior.  So, your best hope is for your SD to work, marry, have kids, anything to have a full life and get out of your hair.  Sorry to be a Debbie Downer but I know you come here for the truth. Best wishes.

Why is a 20-year-old who lives with her mother still spending multiple nights in her father’s home? Time for visitation to end. Also, your first paragraph says everything is fantastic with his kids, then you go on to list serious concerns. I’m not sure you are taking an honest look at the issues here. Don’t marry until you do.

ETA: re-read and see why she's staying at your house - but no, if she wants to be closer to campus she can get an apartment.

Do not marry this man. If you do you will regret it. SD will be his real partner and you will just be his concubine. Everytime you bring up an issue, he will throw up his hands (playing the victim in the middle) or he will tell you that you are jealous (accusations), or he will tell you that you are TOO sensitive (gaslighting) out of one side of his mouth while encouraging you to be more understanding of SD because she is sensitive and fragile (excuses) . He will defend her, make excuses for her, or simply throw you under the bus.

Please put your foot down now & if he does not like it then he needs to leave.

Sorry, but that is my honest opinion.

Your situation sounds similar to mine. I don't think the situation will get better or worse. The question that only you can answer is if you can live with it. Your SO and SD have been cooking that communication style for 20 years and they aren't going to change for you. It's a painful, painful thing to realize. 

And her dad needs to tread lightly... well the real world is ‘t gonna be as understanding to her and will tell her to suck it up.

he lives in your home and doesn’t give you the respect of sitting at the head of the table with him even after you have cooked dinner??

no wonder you feel unappreciated and resentful of him. He claims he wants to keep everyone happy but really thats a smokescreen to keep precious happy

It gets worse after you marry, so you better have this nailed down. My SD's are in their 40's and still acting like daddy owes them something. Things are fine now, but it was a long road to get here.  I have no relationship with either.

Basically, your SO is more afraid of losing OSD than he is of losing you.  She is boosting up his ego, she knows it, and he loves it.  Perhaps he is taking you for granted that you won't leave.

I would expect that my SO would want to see his children, and you should too. I would not expect him to favor one over the other two to the extent that they feel excluded.  That's a lack of character I wouldn't accept. The oldest knows how to butter him up, and that he allows shows he is a weak man.

You need to resolve this way before a wedding or you are going to have a miserable marriage. OSD will still be his #1 priority, and you and the other skids will be fighting for crumbs.

My SD is 40 and in many ways is independent and successful. Yet she feels entitled to control her father--not every day and not about everything. And DH hangs on every word she says. SD said to try this recipe. SD said to use xyz to get out a carpet stain. SD said to la di da di da. She has advice for all of us, all the time, and expects to be involved in decisions that are none of her business.

Mini wife with adult spousal status. DH is afraid his kids will just one day stop loving him. He knows it's ridiculous, but there you have it. I can't stand him running after her like a love sick puppy. She and I actually do fine together, when he's not around.

DH has been open to discussion and has somewhat modified his behavior when I've asked for specific things. And sometimes SD will punish him for not obeying her command. But unless something actually affects me, I don't say anything. It's taken a lot of years to get to this point, and sometimes I still can't keep my mouth shut or my eyes from rolling around in my head.

Figure out what you need. If you need him to make changes, ask for them. But do it before you marry him. If all you get are excuses and if he's not willing to meet your needs and make changes, then you know where you stand--and I'd not be willing to accept part-time wife status.

Maybe it's time for the 20 year old adult to find her own place. Allowing her to continue to come as a "houseguest" when she is an adult. Thirty minutes is not a long drive. If she's crashing at your house out of convenience, perhaps there need to be some ground rules that she doesn't come in and disrupt everything in your home. 

Also, I hate the "I'm just trying to keep everyone happy" line. I think it's such a cop out. I told DH that his job is not to keep his children happy, it's to parent them and parenting means helping them learn how to be likeable people, not demanding, entitled, ungrateful jerks. 

From what I have experienced and read on this board mini-wife dynamic and guilty father syndrome are horrific. I was involved with a guy for two years or so and his adult kids were horrific. His daughter is like 25 years old, has supposedly been in college for 5 or 6 years yet is still a freshman, possibly a sophomore. She doesn't provide him a transcript because according to her it would be invading her privacy. She verbally abuses him and treats him l Iike crap yet he still pours what money he has into her saying "I have to believe in her." He knows she has forged transcripts to get scholarships from an organization through his job, yet he just turns the other cheek. She disrespected him and our relationship and to calm her he offered to take her on vacation. I did break up with him and had cut him off for months. When I finally did hear him out he actually offered to purchase a new house for us and do other things for me financially if we could just be together and accept her no matter what. Smh. I recently found out his other kids are just as lazy and pathetic as she is. Their hands are always held out looking for handouts. I finally came right out and asked him what the problem is with his adult kids that none of them work enough to take care of themselves. He never really gave me an answer but eventually made up an excuse saying people from the area of the country he is from are uneducated and live in poverty. So basically no accountability on their part at all. He even went on to say people who work and take care of themselves are just "lucky." Basically saying his adult kids are unlucky. Again no responsibility or accuntability on his adult kids part. Victim mentality at its finest. I would NEVER be with him again under such circumstances and had I known the truth about him and his lazy adult kids, I would have NEVER given him the time of day

It is your house and you have every right to make house rules. I would start by making a curfew so she doesnt disrupt anyones sleep - non-negotiable. You have your own kids to consider. Start disengaging. If she requests certain dinners or snacks tell her to ask her father or tell her you already planned dinner. Tell her to stop slamming doors.  I wouldn't discuss this with SO just do it. Its your house. If she doesn't like it she can stay at her mothers. Its only 30 minutes drive to school not 2 hours.

WILL harm your relationship when you force the issue.

Daddy feels guilty and "...wants everyone to be happy." That's his first mistake. IT's not possible. As a couple the GOAL OF EACH MEMBER in that relationship is to keep each other HAPPY. If he treads lightly with her, and doesn't want to hurt her feelings etc. he will STAY THAT WAY. SHE WILL ALWAYS BE FIRST. IT WILL GET WORSE WITH MARRIAGE AS TIME GOES BY.

FYI-when he asked her to leave after 4 days, I will bet my bottom dollar that he MADE YOU THE ISSUE or REASON why SHE HAD TO LEAVE, because he "...wants everyone to be happy." 

My DH did this for a long time, and it was just a matter of time until it all exploded in his face. Adult SKs saw it and him for what he was, and I did too. He tried the, "I'm stuck in the middle." Yes, he is the "monkey in the middle" and he placed HIMSELF SQUARELY IN THAT POSITION!

I'm in a similar situation. I had never heard the term mini wife before, but it explains everything! My DH is truly a wonderful man, but so emotionally needy. I, on the other hand, am very "not in my space" type of person. From the very start of our relationship, (been together 15/married 13) I noticed how "close" he and both his daughters were. It made me cringe-the hugging, and sitting close on the couch with their heads on his shoulder, the baby talk and texts, etc. But I always felt it was my fault for feeling "jealous" and it was his way of being a father. Who am I to judge? . Anyway, SD #1 is now married and had a child so she is normal. SD #2 is the problem. I've written before what my issues were, so I don't want to go into everything again. For those of you who are dealing with the mini wife syndrome, my advice is to think twice before marrying. I wish I would have. This has been a thorn in my side the entire time I've been with him. I try to ignore things, but it's always there. I am going to a counselor tomorrow and will report back what she tells me so it may help some of you. I was just glad to see there is really an issue and not just in my head. I've never felt #1 in my DH's life. Also, I was/am not financially able to live on my own-another regret of not getting an education so I could be independent. I married at 18, (ex was 25, so do the numbers-he was 23 and dating me at 16-wth was wrong with me?) had my three children, divorced after 22 years and finally had the guts to walk away. Needless to say, I was free from a domineering, mentally abusive situation, but didnt have the financial support I needed to survive. Met my DH and thought I was in love -(I was in love with the lifestyle he provided) We get along just fine, but more like a brother and sister. I can live with him, but the whole mini wife thing is really horrible to deal with. Run before it's too late for you! Who knows what I will do after my counseling. 

The residents in the home take precedence over related visitors.  Your DH is a ball-ess wonder if ever there was one.

Time to set the very clear standards that will govern SD-20's visits including prior request and approval from you for each and every visit.  DH's behaviors must not change to isolate anyone in the home over the visiting adult SD, PERIOD!  SD will not be catered to. She will be treated as any other member of the home with the same responsibilities as anyone else while she is present. She does chores, picks up after herself, cooks, cleans up after dinner, etc, etc, etc.....

This is not her flop house and refuge from real life.  Family members have responsibilities to everyone else in the family..... 

I would not tolerate DH turning into a juvenile "look at meeeeeee" PITA when SD is present, or at any other time for that matter.

Good luck dealing with your severely lacking in maturity DH and his invasive prior relationship failed genetic experiment.

I say it doesn't matter if it gets worse -- it might, but it won't ever get better than it is now. Here's why I say that:

Right now, you're about as in love as you ever will be. You accept things now that will drive you mad after you're married or going on Year Ten. And when Year Ten rolls around, you won't look so hot, he won't look so hot, the newness will be worn off, etc. So what I'm suggesting is that right now this doesn't seem that bad because you have all the new-relationship stuff going for you. When that's gone, all that's left is him and his daughters snuggling on your couch while you wash the dinner dishes.

Think about it. By marrying someone with kids, no matter what you complain about, there will always be a helpful friend reminding you, "You knew what you were getting into," and "But they are his daughters!" or my favorite, "You have to be the adult in the relationship with his kids!"

I'd take a pass on this guy if I were you.

.... You, the marriage or both.  In my case, I got so frustrated seeing the two young men treat their mom like crap and treat their disengaged dad like gold.  However, if I tried to speak up some or interject, it caused worsening issues with my wife.  We barely have survived this and are trying to come back from relationship life support.  We both failed to settle on real grodundwork and this is the result.  That and many other behaviors/issues have brought me to my knees and though I love her with all my heart, if the adult "kids" continue I am about to the point of kissing her, divorcing and moving to a new city swearing off for good ANY woman with kids.

Believe me, it gets worse.  I have been with my SO twenty years now, the problem with his tippy toes princess has never gone away.  It has been a cancer that has killed our relationship, it is dead in the water.  I feel I'm only in it so she doesn't get her way - to split me and her Dad up.  Thirteen years into our relationship he took out a loan for her behind my back as she had got into trouble with her mortgage.  She has always been a very "wanty" girl and had overspent.  The trouble was, he took money out of our joint savings without my knowledge and rode in on his white charger to save her.  I found out three years later as I questioned him as to where our money had gone.  Even now he says she repaid the loan but there is no evidence to suggest it went back into our account.  He also felt guilty for giving tippy toe princess money so also took our joint money to give to his other daughter behind my back.  As you have already been told it is a symbiotic relationship they enjoy.  A very sickly, sugary pretence that oozes saccharine sincerity.  And I can guarantee you, beyond 100% that things will never change.  You can try to talk sense, explain how you feel, ask for consideration, respect etc etc. but he gets far more adoration from his daughter and that is his life blood.  Save yourself for someone who genuinely wants to treat you fairly.

" ... the problem with his tippy toes princess has never gone away.  It has been a cancer that has killed our relationship, it is dead in the water ... "

So sorry to hear this. I'm in a similar boat. My past battles with DH over his spawn have definitely taken a toll. I've gotten older, too, shockingly, and some combination of step-battle scars and--age? wisdom? exhaustion?--has resulted in the departure of the last whiff of romantic vapor from my blood. My DH is not a bad guy, but his loyalty to SS has resulted in his repeatedly failing me in the past. I still care for him, and hope I am successful in my efforts to treat him well. This careful regard is mutual, I think. And our relationship is still very much a marriage. But I do not trust him. I am ever aware that he can turn on me with a snap of SS fingers. This has made maintaining our marriage quite an effort. I manage, but it is work and I have to constantly remind myself that doing this work is a lot better than the likely alternative. And I am luckier than some, who really ought to get out.

My 20-year-old self would have been shocked and disbelieving at my present jaded cynicism, but there it is.

Ewe.

He gaslights you about it? Double Ewe.

I hardly ever say this. Typically I would suggest counseling and communication. But in this instance, its not too late as you are not yet married. I am married and DH and SD14 have MOSTLY healthy boundaries/respect for boundaries. She is very clingy and yet she has her friends and a social life that doesnt always have to include dadeee.

I sometimes think she acts helpless and askes ridiculous questions for attention, but she does the same with me, too.

Your SD is definitely faking it and if you get married it will get worse and her fangs will come out into the open, because you will then be stuck.

I think, from what you posted, that you are way way way beyond having her stay someplace else. Your issue is bigger. Your SO...he doesnt want a wife, he has OSD.