The incest tone

I don't remember the characters names so I'll refer to them by their real names.

Brittany clearly fancied her teacher, who in turn was actually her brother. She also seemed to have a bit too close of a relationship with her dad (the self-defence scene). I'm just wondering why? Did the writers want an incestuous themed to represent something? Was it supposed to be somehow linked to the rape? I just didn't really see why it was necessary.