It's not easy, believe me.

I'd think about who the parties are for. How close are you to the person for whom the baby/bridal shower is for? How close are you to your grandpa? If this does happen to be your grandpa's last birthday and you didn't attend, how would you feel?

If you want to go to your grandpa's party, you could talk to your boyfriend's mom and explain that unfortunately your family plans everything last minute, and as much as that may drive you crazy you don't want to miss this birthday party. On the other hand, you could tell your family that you have had plans for the baby/bridal shower for months and you are unable to change. Good luck, not an easy decision!