Not sure if this is a long shot (others would know better), but is it worth her getting some counseling? Just thinking she might have some internal issues that need to be addressed with not having her mum, dad remarried and away and then being raised by grandmother and aunty?

Quite often I've found when children act out in this regard they are missing having consistency and someone permanent in their life. The older bf can be the whole "replace a man in my life". Do you have enough of a relationship to just talk with her? Or is there too much distance there?

hun