Maybe he needs to be going into more detail on what happened in the prior relationships

It also might be good to understand what his daughter's plans are.. because if the ultimate goal is for you to move in.. get married.. you need to get a feeling for how your life would be with his daughter.. in the home.. out of the home.

Right now, she is being civil.. and maybe that's all that she needs to do.. but is she going to be out of the house?  because when you would move in.. I would not be all that happy with having another adult woman in the home... it ends up being too easy to have conflicts.. and people get territorial.. it will probably not work well.

When will it be. You only have 4 months invested in this relationship.  You must talk to your BF and get his feelings on his DD and her being disrespectful to you.  First question is, do you really care about SD,   You can not make SD to be your friend,  all you can hope for is SD respect you your position in the family.   It's up to you to go forward with this relationship.  
Certain thing you can change, others you can not, every person have different breaking points.  Something  acceptable to me, may not be acceptable to you. That's life.. 

If you can't get this worked out, and it must be worked out.  Then you should be thinking of leaving this 4 month relationship . You don't need years of fighting 

It is obvious to me that your boyfriend tries to shut you down and deny there is a problem with his daughter, because there was a problem with the previous girlfriend, probably caused by SD21 which led to the end of the relationship.  He doesn't want the same thing to happen again, and so is in denial about it and gets defensive.  What should happen is that he acknowledges SD21 is treating you poorly for no good reason, and does something about it.  What I think he should do is invite her to leave and get her own place if she is not prepared to be civil to you.  She is well old enough to not live with her Dad any longer. I have an SD who is 29, and very nearly caused me and DH to split up, 2 years ago.  These young women have too much power in their fathers' lives and should attend to their own life and leave their fathers to get on with theirs. 

Red flag!!! Red flag!!!!

You are wise to be concerned. Your BF shutting you down and not letting you really discuss the matter with his DD is concerning. It's damn unpleasant to be exposed to someone who acts like that. Your BF is not having your back and that's not OK at all.

Don't make the mistake of waiting until she "gets used to you," or "grows out of it," or whatever other excuse your BF might give to shut you up. Your BF is proverbially smaking your hand for reaching for the cookie jar when he shuts you down and gets defensive over your mention of your concerns around how his DD is to you. He is showing you that he does not want to deal with his DD so he's trying to shut you up instead. Hell no! Forget the excuses about how she might not like you two dating or that it must be difficult to have another woman come around. She's an adult. Too damn bad that Dad is with someone. She needs to be an adult and deal with it. Your BF needs to deal with his DD. They don't change nor do they outgrow it. Silence is assent. She's being rewarded for being cold to you because your BF is not addressing her. He's already allowed the demise of a previous relationship due to his DD and it looks like he's not willing to change anything. He might be one of those Disneyland Dads who prefers to cycle through women until he finds one who will tolerate an untenable dynamic rather than address his DD. He's likely one of those guilty dads.

Being exposed to that is nothing less than soul-destroying. Ask me how I know. It isn't just the coldness from your BF's DD. That's actually not the issue at all. It's the fact that your BF doesn't have your back, doesn't let you fully speak to your concerns, and shuts you down rather than learn of your feelings and worries around what's going on. I suggest telling your BF that you want to backstage your relationship and you don't want to come to his house as it's too uncomfortable. Also, tell him you need some time to think things over and consider your options because you're concerned about a significant unpleasant dynamic that he does't want you to talk about and you prefer a man who has your back. I would be very honest with him and tell him that you highly suspect that his previous relationship broke down because of the dynamics with his DD and you don't want to sign up for that. Do something now because if you don't take drastic acion and remain steadfast on it, things won't change. Be on the lookout for your BF saying crap to you to buy time. No. His DD is 21, she ran off a previous GF, you need your concerns addressed pronto and anything less is not satisfactory.

As uncomfortable as your BF may be with this topic.  I do think you need to have the conversation.. if he is defensive and shuts down.. that is a clear sign that this is not the relationship for you... you don't want a partner that can't communicate.. even when the subject at hand is emotionally difficult for him.

i would tell him you need to have him listen fully to your concern about being in a relationship where you feel there is a person that could be actively working against you.. that if he can't talk about it.. you can't invest any more time or effort.

This sounds like industrial grade baggage. You've been dating him a year and were just introduced to his 5(five!) children 4 months ago. One is an adult daughter who's already giving you the cold shoulder. Why does only one of the 5 live with the BM? Is it because your SO will do anything to be the "fun" house? Something is off here. You formed this relationship only dealing with him for 8 months. That wasn't the real him, though, this is, and this is weird. 

My suggestion would be get out of it now. Whilst it's still early on. Don't spend months and years mulling over why she treats you that way. She'd treat anyone in your shoes the same way. It is nothing against you personally, it is against the idea of you, the idea of someone else being with her father. And as long as your BF doesn't draw the boundaries with her, she will always act that way. She will always be ruining his relationships, because HE LET HER. And don't feel sorry for him either. He is an adult, an adult should be fully capable of drawing boundaries with his adult daughter and telling her when she's acting rude. Don't stay and wish for things to change, because they won't. Don't stay and wish that you'd be the one to change him and you'd be the one that she accepts. You won't be an exception. Leave now, otherwise you'd be needing years and years of therapy and healing. 

Yes, she has no say in you and her father dating. However, neither does she have a choice but to be reasonably behaved, polite, and respectful.  I would make that clear to all of his kids, aprticularly the kidult SD-21 who should know better and not be allowed to behave otherwise.

Be direct, do not tolerate the disrespect and lip service and make sure daddy understands that you will not tolerate it so he better get it fixed before you have to or no one is going to like you dealing with it.

Lather, rinse, repeat any time the failed family progeny deviate from standards of reasonable behavior and standards of reasonable performance.

Keep it simple.

KISS works.  Particularly when those in the toxic blended family opposition step out of line. Regardless of who they are.

End this relationship .

It a MESS and will be a bigger mess as time marchs on.

I'm glad you came here. If I were to talk to my old self, I would say: WATCH THE RED FLAGS. Do not discard them/brush them off. Trust your gut. Tread carefully. Be good to yourself. 

Keep in mind that you're still in the "Honeymoon" period. This does not get better. 

They haven't even unleashed the beast yet. The daughter is building up her "But I was civil to her this whole time!" innocent-girl defense, and your boyfriend has already chosen to invalidate you and shut you down. Healthy relationships require respect and honest, open communication about the difficult stuff.

Throw this one back. You can move out and move on much less painfully and expensively now, rather than some magical thinking about 'waiting it out" for five, 10, 20, or more years of your precious life. Listen to your gut - it's correct.

Glad you found StepTalk. You can read countless true stories on here about how this exact same scenario plays out. It's not pretty.

5 kids with likely multiple Xs.  Yep, end it.

So, what is happening is probably something like this: His DD acts civil but cold. Doesnt like that daddy cakes is paying more attention to GF/Partner. GF/Partner tears her brain apart examining each and every interaction trying to see where she went wrong. Asks partner, partner shuts down the convo, or gaslights. DD wins. Daddy wins. GF/Partner loses. The triangle continues each new person...and the cycle continues.

Look up mini wife. Read around on here. I have had the shunning in my own home, the being treated horribly, called names. 

Hopefully your partner has your back on this or you back off.

"...she will say hello to me and ask me how I am but it is rhetorical i.e. she does not want an answer, she will either stay upsairs in her room, or sit in the lounge and close the door, and she will only speak to me and acknowledge me when she has too.  No conversation or dialogue is directed towards me, or eye contact."

Identical! This is how SD behaved when I met her at 14. My mistake was thinking she would eventually come around, because she was at such a difficult age. 

I was wrong. Very wrong. I am now 20 years in and to this day, I HAVE NEVER had a one-on-one conversation with SD!  I have never spent even 5 minutes alone with her.  She has never, not once, uttered my name to me. Since she doesn't talk to me there is no need to use my name. 

In fairness, she has also never been overtly rude or disrespectful to me either. I am simply a non-entity. If she must respond to me, like when I ask HER questions, she does so with brief responses that are stated in an unemotional tone and she does NOT look at me when responding.  

The difference in my case compared to yours was my SO and I did discuss it on many occasions over the years. He had a serious heart-to-heart conversation about it with SD once and she essentially clammed up and didn't talk. Behavior never changed. Again, since she was not overtly rude it was unrealistic for him to tell her she had to ACT like she liked me. She was not being rude, after all.

After that, if I brought her attitude/behavior up again my SO's ultimate response was, "What can I do about it?" And in a way he was right. Especially once she turned 18. 

Like many Doormat Daddies he was terrified he'd lose the tenuous thread he had to her if he pushed her too hard.  He felt he did what he could by sitting her down and talking to her, and it had no results. He didn't want to do it again. And so I chose to disengage. 

SD is now a married mother of three and her behavior towards me has NEVER changed. It is still identical to the paragraph above. Thankfully, we live many states away from her now and my SO goes to visit her at her home. I did it once and will never do it again. 

In hindsight, I still think me disengaging was the best thing to do in a difficult situation like this. Sure, it galls me that SD gets to spend time with her father alone and keep me out of her life - which was her ultimate goal. Fine by me. I'm beyond scorekeeping and if she wants to gloat over this (along with BM, who is 100% enmeshed with SD) then so be it. Life is too short to waste it thinking about, being with or worrying over people who don't give a sh!t about you. 

You are only a year into a relationship so you have time to reassess. And your "problem child" is already an adult at 21 and it is also doubtful she is going to change so no hope on that front. 

I do sometimes ponder what my life would have been like if I had met someone whose kids were at least friendly and accepting of me. I have several friends whose stepkids are inclusive and warm towards them. So I know it is possible to have good relationships with skids, and be included into their lives and families - including a role as another loving grandparent to their kids.  

Unfortunately, I don't have that. And never will. 

I know we can internally say that it is skids' loss when they don't get to know us or at least give us a chance. After all, most of us want to think we are likeable, are good people and don't want to cause rifts in families. That can be very true.

It can be equally true that skids see us as unlikeable, threatening, and the cause of their family strife. Therefore, they don't see it as a loss, they see it as a win when they are successful in kicking us to the curb.

You often see people advise to stick by their spouse's side and essentially plow their way into visits, I don't subscribe to that. The reason is simple self-preservation - I don't want to subject myself to a situation where I am treated badly and uncomfortable. Maybe that works for some, and while the skids are younger, it won't work forever.  

While your SD is 16 now, eventually she will become an adult with her own home and her own family. When that happens, you have to realize that this is an ADULT who does not want you in their home or in their lives.  Even if they don't explicitly say they don't want you there (like my SD) it is still very obvious you are an interloper.  You cannot force yourself into another adult's home or interaction with THEIR children when they don't want you there.

Why would anyone sign up for that kind of abuse, just to prove a point that you are a solid couple?  My SO and I know we are a couple. Our friends and (other) family recognize us as a couple. Strangers even recognize us as a couple. If  Adult SD wants to continue to ignore that fact, fine by me.  Maybe she thinks that's a "win" and that's fine by me too. 

What she probably doesn't care about is that I consider it a "win" too, since I have excluded her from my life and have justifiable reasons for doing so. 

BF is playing peace maker blowing smok up your aaaa.   He must pick a side unfortunately.  He can be Mr. Middle.  
'You must in the hext few weeks understand. Your position in this relationship, and family.  You must decide if this is for you or not knowing this is the best it's going to be. Downhill from here.  
Some people can do it, others can't. That is up to you, it's your life. You have one chance at life. 

Perhaps she grew close to past girlfirends and was hurt when her father was no longer seeing them? It sounds like SHE's got issues and you shouldn't make her aloofness to heart. I would, however, be rather concerned about your BF's dismissive attitude toward the situation.  You are voicing a concern and he's shutting it down. That's not a good sign, in my opinion. You may want to re-examine your relationship before you get in any deeper. This aloof daughter may know things you don't.  Best of luck to you!

That's why he try to get the relationship going makeing it harder to break off the relationship.  BF has choices to make.  Basically you or his kids. Sounds cruel. It is also true,  He can not keep trying to make everybody happy.  He's stuck with his kids, you are not stuck with him.  
4 months is not a long time , but  but.  He knew the kids would be a problem. And did nothing about it. Letting the chips fall onto you. That not saying much about BF.  '

IN the end. It's your life,, you choose how you are going to live it. Fighting with SK is not the way to live.