Is this really a battle worth fighting? When DH's parents were alive they wanted their grandchildren to call them "Nammy" and "Pa". No one questioned it. I've heard of grandparents called all different things, including Mimi. Personally, I'd let this slide. It's not worth it. If someone is old enough to be a grandparent I think they've earned the right to be called what they want, provided it's not obscene, of course.
FWIW, I grew up pronouncing *aunt* as "ant", because my family is of English roots. However, DH's family is French, and they pronounce it "aunt". It's not snobby at all, it's just the french influence.
My kids use both pronunciations, depending on which side of the family the aunt is on. LOL!