I'm sorry to tell you this, but there is nothing you can do. All you can do is love and embrace your brother for who he is without her. Don't try to blame things on her, because he is choosing to stay for a reason. Since it doesnt seem that you care too much to try to change her or have a relationship with her..just love him. To love is to respect, don't bash her even when he may ask your feelings about her. When things get bad in the relationship, hes going to need comfort and support in his decision and it wont be from people who have bashed her all the while. So, love him...ask them both to hang out a lot! If he accepts, maybe he will remember what fun is...or if things go badly, he will still feel comfortable talking to you. If he feels like everyone is against her, he may stand by her just because he's a good guy that knows she wasnt given a chance. Give her that chance...sounds like she'll blow it and when she treats you, his sister, like dirt...he might come to the realization sooner.