Originally Posted by sahm1225:
I think he’s being disrespectful expecting you to change how you and your kids refer to her, I am old fashioned and I feel that an adult should have a title before their name, BUT Mimi is a term of endearment and how she wants to be called. It’s not like kids say ‘mrs.grandma’ so why should it be miss Mimi?? 
Plus The kid is TWO. It’s not like he’s old enough to understand
I would tell your sister to tell her husband to knock it off. My husband's mother is called Nana. And when she comes around daycare kids she is called Nana by everyone. She loves it and the kids recognize her as another motherly figure who cares for them. My sister's mil is called gamma. All of our kids call her Gamma S******. My kids have no relation to her or her husband (grandpa w****) other than marriage, but my kids respect them as the matriarch and patriarch of the family. It is not so much the name as it is the statute of the person within the family. Kids are not dumb. They know that the person plays an important role, so for them, the name means nothing, it is just what they call this important person.
I personally would call my sister and find out the real problem. Probably has very little to do with what the child calls your MIL and much more to do with them not wanting her around for some reason or even possibly because of your own lifestyle or her impact on it. I can't imagine having my sister care for my baby and then using a term of endearment as a means to start a fight or cause disruption of that relationship. It doesn't make sense, and if your sister supports her husband's view, I would be wondering what the bigger issue really is, bc something is missing, imo.