I think as much as it sucks I am going to have to just not go. If I am there I am the scrooge to her ruining the fun.
I dunno I got to think on it.
As long as you act upbeat and have fun with your son and do not use that event to try to warm-up to your W, then have a great time. Be nice to her but don't treat her the way you would have in the past "as your W" and let her begin to see how things as a S/D couple will go. Just be nice as if she was one suppose to be a blind date--who turned out to be your cousin. You wouldn't romance your cousin or smother her with attention. You'd be getting back to your sone.
If she has a probalem with that, well then....that her problem, not yours. You will be seeing how often youwill face that same bridge.
After that you've been fun at the party, and has shown your son a wonderful time......if she hasd a problem with that
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!