Advice?!?

So a few months ago my boyfriend was out with a girl that's a friend of his for her birthday, he drove her home because the rest of their friends that had been there ditched out early but when he drove her home she tried to kiss him, knowing he had a girlfriend. He told me nothing happened and when I asked him if he wanted to be with her he said no, and I believe him, his last girlfriend cheated on him and maybe that doesn't mean he wouldn't do the same thing but his reaction to the situation makes me believe him. Now in a few days we're going out with some of his friends and I'm worried she's going to be there (one of my bf's good friends dates her cousin) and I honestly don't think I'll be able to be nice to her, and I'm worried I'll make his friends dislike me because one of them is her cousin. Any advice?