Okay this might be a long story. (didn't know where I should post this)

In my group of friends there is me, my boyfriend, and my bestfriend (a guy). It used to be the three of us and my bestfriends girlfriend, but she dumped him for no reason about 5 months ago. She no longer wanted to hang out with me or my boyfriend and didn't give us a reason either and it hurt me a lot because we had been friends long before they started dating. Well on Christmas she called him up and even though she broke his heart he took her back. He brought her to come see me the day after Christmas (since I had gotten out of the hospital), and I wasn't mean to her and neither was my boyfriend, but I'm not sure I'm ready to hang out with her again. My bestfriend is like a brother to me and I couldn't stand to see him so heartbroken over her and for her to just not want to hang out with me anymore because I was such close friends with him. While they were going out before, she would get so jealous of me hanging out with him and thought we would do something with eachother. I don't trust her at all anymore and from things I have heard (like she never loved or liked him in the first place) I don't like the idea of them going back out. I promised my friend I would be nice to her, but I don't know how long I will be able to go without letting her know exactly how I feel. I think my friend could do so much better than her. He knows my feelings about her, but we are always hanging out with her. I think she is going to break his heart again, what should I do? Should I keep being nice to her for his sake?