I don't think that's your problem. This is your boyfriend's problem. It is his fault that he could not raise his daughter as a polite person.
I understand that you are worried, but not worth it. You behave with dignity towards her and you do not have to tolerate her bad attitude towards you.
You need to tell your boyfriend that.
Say that you are ready to communicate with his daughter and you will be glad to see her as a guest, but as soon as she starts behaving rudely, it will be better for her to leave, because you do not want to tolerate it.
She is not a little girl who can be forgiven. She is 21 years old and she must be responsible for her actions.