Growing up, nearly everyone I knew had a Meme and Pepe (mem-may, pep-ay) and a Grandma and Grandpa. (One set of French Canadian grandparents, one set of not French ones!)

Maybe instead of Mom-Mom, she'd be okay with Meme? What's her ethnicity?

Other common Grandmother names:
Grammy
Gamma
Nana

Have you spoken to her at all about it? Maybe she'd be fine with something else, you never know 'til you ask!