is it worth it?

I know this will sound crazy, but I get so upset about my boyfriend's girl cousin. She is my age and she is super fake and dislikes him and I. But yet ar random times she gets all fake and sweet and tries to be buddy buddy/close with her cousin/my boyfriend. He always complains about her and tries to avoid being around her, but when he cant control it and she gets all fake and close, he falls for it and is nice and close back. It makes me soo mad because I listen to him complain about her yet he'll turn around and be all sweet to her when shes fake and all buddy buddy to him. I confront him and he just says he can't help how she is and what she does. Should I grt mad about it or just leave it be? It is soo hard to ignore it when it happens.