step grandparents

My stepmother has been very firm about not wanting to be called grandma. She has been with my dad since I was 6 years old so I really feel like she is part of the family. It wouldn't bother me so much if it weren't for the fact that my mother has stage 4 breast cancer and my husband doesn't have much of a relationship with his mother who left when he was a baby and lives across the country so he's only ever spoken to her on the phone. Basically there will come a day when my stepmother is the only grandmother my daughter will have. It bothers me that she won't take on that roll. Am I overreacting to this? Did anyone else have this issue?