I hate my boyfriends sister?!?!?

Where are the parents in all of this? I'm uming you and the BF and sister are all young and living at home. Sounds like a messed-up family. Lay it on the line (what you wrote here), if you really think this guy is worth it. Tell him you can't be around her (and/or his family) unless she changes her attitude. And don't forget, this BF of yours doesn't sound like he's being particularly loyal to you in all of this. You choose to be around his family. Read the writing on the wall, sister.