Allow my fiancé’s STEP mom to be called GRANDMA?

COMMENT YOUR REASONINGS :)

So I’m going to give a brief background of this lady. I don’t even consider her to be a step mother, at all. My fiancé’s mother has passed away, since then his father has remarried. This “step mom” has been in my fiancé’s life since he was a child (his mother passed when he was 4) Anyway, this lady doesn’t do anything for my fiancé except bitch at him and talk shit about his mother and her family. It has gotten so bad, that his mothers family cut ties with him and his father...so I’m asking because I’ve been told it’s messed up if I have my twins call her grandma, but ive also been told I’m messed up if I don’t! I need some outsiders opinions.