Do I go to this gathering, or leave it?

My opinion: Go to the party anyway. Don't let her negative actions ruin things. If you have a strong personality then you should be able to sidestep her bad actions. You might even have to say a few words to her that she wouldn't like. So what? She's been doing a lot of stuff that you don't like, and you do have a right to enjoy yourself in spite of her. Don't let her have the upper hand.