My Boyfriend's brother's girlfriend doesn't like me (is being a b***ch)... Advice?

Anonymous
I've been dating my boyfriend for a month. Prior to this, I spent some days are his place even before we started dating.

He lives with his brother and his brothers girlfriend.
The first time i came to his house, I met his brothers girlfriend in his room which was quite unsettling to me... She left immediately I came in.
I'm naturally quiet during the first few weeks of being in a new place with new faces so I was a bit reserved... His brother was trying to bond but the vibe I got from his brothers girlfriend just made me plainly uncomfortable.

Just talked to my boyfriend and it turns out she was good friends with a girl who people expected him to date before he met me.

That can't be enough reason for her salty behavior though... I honestly thinks she feels threatened that I'm claiming her spot as the family's centre of attraction or she probably likes my boyfriend because there are hints that she actually does (She's called him daddy once, tffff?)... What really bothers me is I'm trying to get to know my boyfriend's brother and she's just being difficult and affecting him which in turn bothers my boyfriend because he holds his brothers perspective high.

My boyfriend has made it clear that he's sticking with me and they should try to adjust but I don't think he has made it clear enough. This is total BS however... Please I need suggestions on how to talk to my boyfriend about it without bursting out and how to get along with her without kissing her ass.
My Boyfriend's brother's girlfriend doesn't like me (is being a b***ch)... Advice?
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