Answer Without having a great deal more information, I cannot answer this question specifically. However, I can raise some questions and speak generally to the issue of relating to a lover's children. I assume there has either been a divorce or death. I also wonder when the divorce or death took place. In the case of divorce, I would want to know how involved in the father with the girl.
Children harbor the fantasy of their parents re-uniting.
Even when the parent dies, depending how they felt toward their father, whether they idealized him, they will have difficulty accepting someone coming in to take his place. Their behavior is often geared to frighten the substitute off. She may be displacing anger that she feels toward either her mother for getting involved with you or her father for not being there, or both. It is also possible that the daughter is acting out some general anger she has about her circumstances. You are the safest one for her to get angry at.