In my opinion, it doesn't matter if you're married or not. If you two are in a committed relationship there still should be some respect here. Just because you aren't married doesn't mean you should just go ahead and lay down like some old door mat.
If I were in your situation and that was going on, I don't know if I would be able to hang around. You said your boyfriend was devastated when she left him. Your boyfriend's mother still thinks the ex wife is awesome. It also sounds like the ex wife thinks it's all right to be a cheater and still mingle with the family of her ex. Personally, I wouldn't have the nerve. I'd be embarrassed as hell. I also don't know of any self-respecting woman that wouldn't be able to move on with her life instead of hanging around the family of her ex, except for keeping it cordial for the kid's sake only. Calling up my ex MIL to shoot the breeze outside of the kids sounds desperate to me. Move on with your life. I also will respect his new woman. I when you have respect for yourself, you tend to respect others.
Isn't she getting married? Does her fiance have a mother? Why doesn't she ask her? Also, what kind of self-respecting man would let his soon-to-be wife plan a wedding with an ex MIL? He sounds like the door mat!
I don't know, but that sounds like a big mess an the potential for some hurt feelings, mainly yours. It sounds like no one really gives a rat's behind about your feelings, so that's something you might want to think about.
EDIT: I'm so sorry everyone is harping on an on about you not being married. That wasn't the question. I tried to answer your actual question the best I could. Good luck to you, and yes, you have a right to be upset. No one can take that away from you. Those are your feelings.