I'm not going to invite my mom or my grandparents to my wedding
That seems harsh. Why don't you want to invite them to your wedding?
I don't think they'd be very much fun to have at the wedding.
I am not sure that's a a good reason to not invite them. When people are having a big wedding, they usually invite their family, especially their parents and grandparents.
But I don't want to.
You may not want to, but I'm sure the rest of your family that you do invite to the wedding will want them to be there. Weddings are meant to be spent with your friends and family, even if they aren't very much fun to be around. You don't have to spend the entire wedding with them, anyway.