My uncle's wife is my aunt and my aunt's husband is my uncle. Well, I call my uncle's wife an aunt, but you're right, your aunt is your mother's sister, but also your uncle's wife. It's like they're going to call their aunt's husband uncle. Your family's culture is very strange and alien to me, so I can't help but take a sympathetic position with your aunt (writing uncle's wife didn't look good.
Even if people didn't know if it was your aunt or uncle who has a blood relationship, it would be pretty obvious that you wouldn't refer to your own grandparents as my aunt's parents.