Just my humble opinion, but I think holdong on to a grudge for 3 years is a bit much. She would be showing diplomacy by celebrating the birth of two innocent children. It's not like this happened last week. His sisters apologized for whatever part they played and now is the time to put differences aside for the sake of the babies. That is not weakness, it's maturity poise. And since she knows his family is important to him, it won't hurt her to support him and go to this party. These are adults after all.