Really? Why can't you just go and make your grandma happy? I go to church when there's a memorial for my grandparents' death. Not because I believe in Christianity, but out of respect.
I did the same thing for when my grandfather passed away. And for Palm Sunday for some reason.
The people are nice and everything but honestly I didn't want to go for any other day that wasn't for my grandfather. I really see nothing in going to church except for loved ones in their memory.
You should go anyway. You never know when something might happen, you'll lose her and go "Why didn't I just go to make her happy before she left us?"