My DH's parents split up when he was at uni, and both remarried. This all happened before I knew him or them. When we had DS1, he effectively had 6 GPs. We talked with all GPs about names ie who was going to be "Gran", "Grandma", "Nana" etc and DH's stepmother ended up being "Nana", which is what our DS have called her ever since.
We saw FiL and sMiL in the summer (they live several thousand miles away). DS1 and 2 are now in their teens and at one point during our stay, sMiL said to me "they don't need to call me Nana any more, they're old enough to call me by my first name". I was a bit taken aback as she was saying it only in relation to her, not to FiL who is still "Grandpa." In other words, she was suggesting that as she wasn't their "real" grandparent then she should be called by her first name. My DC have only ever known her as "Nana" and, although of course they understand she is not a blood relative they consider her to be as much, if not more, of a grandparent than DH's mother. I didn't pursue the conversation at that point, and the DC called her "Nana" for the remainder of the holiday.
Then this morning the DCs got a nice chatty letter from FiL and sMiL, signed "Grandpa" and "(first name)". I feel really upset that she's distancing herself from them, and that maybe she doesn't consider them to be her "proper" GC - AIBU?