Shes YOUR daughter not your parents. Go because you have the right to be there ! it will mean so much more to your daughter that you attended, and if it doesnt then well, you need to get her back from the evil JW clenches.
Repeat after me (my mom's not a JW, but I have this branded on my forhead) "Mom, I just refuse to argue with you. I'm not discussing it." Then don't. Let your daughter see her grandmother treat you like crap. If she's not stupid, she'll catch on. And she'll be grateful that you were there, and that you didn't fight with her grandma.