Mother in law refuses to be called grandma?

I have recently announced my pregnancy to the whole family, and my mother in law mentioned the other day she refuses to be called grandma, she will just have the child call her by her first name. I am not okay with this, I want my child to know who she is and how they are related. But I don't want to offend her either, she is very self conscious of her age and I don't want to upset her