I, too, would go to your grandfather's birthday, specially when his health is failing and this could be his last birthday party.. If you don't go, and God forbits he won't have another birthday for you to celebrate, you'll regret it for life.

Your bf mother should understand. The most negative thing she could talk about you not being able to attend the baby shower is that you put your family first - and that, in my opinion, is not a negative thing at all.