Your sister has made it clear they will still be in her life, so it is up to you to try to deal with/get over what has happened with your ex and his family. When I've been manic I've done some terrible stuff, not saying you did but it happens.
You mentioned you have a son, don't you want your son to grow up and know that parents break up, and while they don't always get along, they can come together and attend family gatherings such as this?
I am bp also, and sometimes we are too sensitive, think "it's all about me", read too much into what people do and say, etc.
Maybe this would be a good time to break the ice in a social situation and put old demons to rest, smile and nod for a few hours for the sake of your son and sister?
I'm sure you don't want to miss your own sisters wedding next year over this.
Just think of all the time you are already spending emotionally fretting and being upset, anticipating missing the wedding because you are pissed about something that happened 4 years ago. Maybe you could put that time into learning how you can get to that wedding, learning to forget past hurts and wrongs?