First of all, your 21-year old step daughter is not a child??she is a grown woman. She has had twenty years to have a relationship with her father before you came along, so you know her claims are unfounded.
It is difficult to be the scapegoat, especially because when it comes to your boyfriend's relationship with his daughter, you are the outsider. I know what this is like because I am a step mother. No matter how old children are, they can still become very upset over their parents' breakup and jealous or cruel toward their parent's new partner.
Do your best to empathize with this girl's feelings, but do not accept her blame. In time, she may come to accept you if you allow her to have her feelings now. She may come around, she may not.
In the face of her blame and anger, hold fast to the truth: you did not come into the picture until after her parents broke up.
