Should I be nice to her?

Okay so here is the situation. I really like my best friend’s brother. She knows and she wants us to get together. The problem is he has a girlfriend. He is always saying that he wants to break up with her but she just found out that she has cancer, and he thinks it’s a bad time to leave her. I know he likes me, and he has always been really flirty with me. About a month ago, we messed around a bit, but I said I was uncomforable with it because he has a girlfriend. Since then he has been avoiding me and when I do see him, he is not his usual flirty self. Now his sister (my best friend) is having a party and wants to invite him, but doesn’t think that he will come unless his girlfriend can come to. She said she would tell him no if it made me uncomfortable. I don’t know what to do. Part of me thinks that being somewhere that his girlfriend is and trying to be nice to her knowing that I have messed around with her boyfriend and that I definately have alterior motives is wrong, but part of me thinks that I am just overreacting and that as long as they are together I should make an effort to be nice to her. Does that make me a good friend or a sneaky backstabber?