How can I like/get along with my brothers gf?! =/?

You don't have to like her, but take the high road when it comes to her. When she is out of line, tell her. Say "What you are doing is disrespectful and childish. How old are you? Grow up,get some therapy for your problems, because clearly you have boundary issues. How you act is not funny or cute. You are embarring and its sad that you don't see it. I put up with you out of respect for my brother and if it were not for that, I would laugh at little people like you. You are a joke and one day when he realizes it you will be dust." say it calmly without swearing or cursing. The more control you have the more power the statements will have. Yes they are rude statements but maybe she will finally learn to act more appropriately. As for tomorrow, don't cause a scene. Trust me when I say everyone will notice whether they say it out loud or not what a pathetic loser she really is. Let her destroy herself to marrow and after that, then let her have it.