If the girl has been greatly traumatised by the previous divorce or separation and disturbed by the new arrangements, she may well be expressing, herself, intense needs for affirmation, identity, value and affection. This can then become an explosive mix, with the stepfather sliding step by step into an abusing situation with a girl who is also making some very encouraging gestures. The situation is further complicated by the possible history of the girl's own mother. If she knows her own mother got pregnant with a boyfriend at the age of fifteen and had sex the first time when she was thirteen, that may also help create a culture where her daughter is half way to the point of sexual experimentation before she starts. Role modelling on parents is a very powerful process.
Children often oscillate between modelling and doing the opposite.
Often they do the opposite of what their parents want or expect of them. If their parents have changed their own attitudes, opinions and lifestyle since they were teenagers many years ago, then their rebellious children now may find great amusement in justifying their rebellion as mere role modelling.