i just told people "it's immediate family only and a few of our best friends" - obviously this wont work in your case... grams is not going to be happy that she's family and that you may have invited friends over her. just out of curiosity, why isnt she invited? you cant squeeze her in?
my own grandmother is mad at me for getting married, and is too sick to travel anyway. but if she had been supportive and wanted to come, we wouldve figured out a way to include her. we wouldnt pay for her trip or anything, but we would've cut someone else from the guest list in order for her to go - i shouldnt talk though, we sent out 300 invitations, and have 100 people showing up in mexico next week, so my wedding is nowhere near the "small and intimate" that i originally imagined.