First off I am sorry to hear about your parents getting a divorce.
Secondly this girl sounds like she has lots the plot!
Sounds like you have been really good to her and she has done nothing to deserve the kindness you have shown her.
Your BF really should stick up for you and tell his family that he will not tolerate them treating you this way.
Sounds like you are better off without them anyway if that is the kind of people they are, just concerntrate on you and your BF because Im sure his family will need you before you need them!