Smurfette, I am sorry for your loss. Grandmothers are special.
What was said to you at your grandfather's service caught my attention. "my mother asked me to leave the wake so I wouldn't make the brothers and sisters feel uncomfortable".... I am sorry, but this is horrible. You go there and hold your head up girl. Their comfort is irrelevant right now. Any mention of your faith or your religious choices, is out of place.. and these people should know that. I hope you don't stand there and let them talk down to you, or use this as an excuse to preach at you. That is the most cold-hearted and un-loving thing I can think of to do to someone. It is selfish of anyone to use someone's loss in this way.
While I think it would be an inappropriate time and place to do battle with the "brothers and sisters".... don't let them bully you. This service is for YOU not them. If you can, bring a close friend, so that you don't find yourself standing alone in a corner, feeling even more vulnerable.
My family, will think of you this weekend, and wish you comfort, and strength.