Do they attend the same school? Are they in the same grade or are they year apart? If so, there may be peer pressure issues at play here, but you just don't see it because the playing field is at school rather than home.
If the girls already belonged to "cliques" or there is "playground" politics at play, your two daughters may never be friends.
While parents like to believe they know what their kids are like (behavior wise) when they're away from home, there really is no way to be sure. Perhaps one of the girls is into a "faster" lifestyle such as experimentation with boys and drugs, or is more focused on social climbing, and the other girl isn't, gravitating more toward academics, self-development etc. What you are witnessing maybe a battle of wills and values...or a power struggle for dominance in the pecking order of cliques at school. Only the two girls themselves really know the answer to that question. They may or may not freely share with you if you bring it up for discussion. But don't count on it. They're probably biding their time while trying to spare you and your boyfriend undue angst until they know where your relationship is headed.
While what I present is just an example of one possible dynamic, whatever the differences are, such a varience in lifestyle goals can really be divisive. And to force the two to be friends will only cause the girls to feel resentful and possibly in the longrun sabotage your relationship with your man by creating an atmosphere of divisiveness.
To add complication to the issue, because you do not mention if marriage is a possiblity or in the works, there is no incentive for the girls to take the living arrangement seriously or to feel a need to work at making the relationship work. While you have chosen to live together as a family, it might still not seem to be "permanent" enough of a move to make them feel obligated to work together as a traditional family might.
There is also the melding of two developing value systems to consider too. Your girls are at that magic age where they are starting to decide "who they are" and "what type of person they want to be." Remember prior to the one girl becoming your "stepdaughter" she has been influenced not only by her clique of pals at school, and her father(your boyfriend, but also her biological mother.
There is no way to know what the girl and her mother talk about when and if they are together...and no way really for you to know if she's less than happy with this new living arrangement. The one thing you can count on is her loyalty to her own mother. Short of her mother being the biggest witch in the world, she's going to probably hold her mother and what her mother values (morally, friends, life choices etc.) in high esteem. Don't discount the possibility that she's sized you and your daughter up, and has decided she might just have the power to put an end to a situation she and possibly her mother don't approve of.
I agree with another poster. Encourage your daughter to make her own friends and put little credence in the ill-treatment and exlusive behavior of the other girl. Do not put pressure on them to be friends.
From my own experience in trying to bring two opposites together (growing up, I had cousin who had to come live with me because her parents went through a very nasty divorce) it is nearly impossible to bring two ideologically different people together. Especially in a situation complicated with issues such as divorce or separation which was me and my cousin's case.
We never got along since birth as her mother had a very different value system than my mother. She resented my mom and me... she thought we were "boring and geeky" since my mother was very into her faith, very strict with us morally speaking.
While with us, my cousin lived a "secret" life and had all of our common friends turn on me. She too was sullen and moody. She never talked in full sentences to anyone. I'd hear about what she thought of us at school where I was labled and treated like the goody-goody thanks to her because I didn't agree with, nor find any interest in the activities she was into.
Our extended family couldn't understand our relationship which was wrought with tension and fights. Though her mother asked my mother to help out, her mother and her made my mother and I miserable...it boiled down to ideology. It was a turf war but on a mental level.
While there was no battle for the affection of a man in our situation such as yours, there was a power struggle between the two moms as to who was living "right" etc. I think aunt felt guilty that things in her life got to a point where she needed to send her daughter and son to live with my parents. A loving decision, but she wasn't happy about it.
In your case, your boyfriend is the central focus of this struggle between the women in his life. The girls may be wondering `Who is going to win him and his affection..permanently?' Remember it is the nature of children to want their biological parents to be happy...but happy together. Even if separation and divorce were unhappy..they always hope someday their parents will work it out. Perhaps your boyfriend's daughter is threatened by the move in...because now reality is sinking in. Even if her mother has moved on to a new relationship, she could still have issues.
In my story, it was a long miserable relationship through high school for us. We didn't even talk to each other at holiday gatherings! We only talk once in a while now. But it's strained.
In your case I say save the family building exercises for when and if you know this is arrangement will lead to a permanent decision. It will be easier (emotionally speaking) for everyone involved. Mostly because then you'll know the emotional investment is worth it. Otherwise, expect the girls to go their own way, and just remember to be your own children's best champion, though you mean well to show love to the other girl. I just say be wary and keep an open eye for your own!
You have no way to really know
