Slept with my best friend and he lied...?

I'm so upset. I've been best friends with a guy for over 5 years. About a month ago he came over to my place and we were watching a movie, one thing led to another and we ended up sleeping together. I was under the impression that he wasn't with anyone. I found out later that he was still engaged!! I'm heartbroken but it's what happened after that hurt the worst. We had emailed back and forth about our feelings towards one another and his fiance found out and sent me a nasty email telling me to stay out of his life. Obviously I know he isn't really my friend or he wouldn't have done this to me. What I'm wondering is if I should tell his sister. I'm really really good friends with her and almost his whole family but as far as I know, they don't know about yet because they're still my friends on facebook. I'm just so hurt and this isn't fair. I'm not a homewrecker. I would have never let it get that far if I knew he was still engaged. Plus I feel bad that he would do this to his fiance. Should I tell his sister what happened before his fiance gets to her (they're friends too), or should I just hope that it never comes out? It's bad enough I'm feeling the pain of betrayal from my best friend but I don't want to lose his sister and his other family as friends either. I'm so torn and so heartbroken :(