Anonymous wrote:
Anyway, I think it matters less whether she was a mother to you and more whether she'll be like a grandmother to your kids.
I completely agree. My grandfather married "Sue" after my mom was already an adult and married herself. Sue was never a mother to her, but as "Grandma Sue" she was very involved. Looking back, I can see that she, not my grandfather, was the one to send cards and buy presents. And now she as "Great-Grandma Sue" she is a loving figure for my kids.
If you hope that she, alongside your dad, will be a grandparental figure to your children, I'd encourage you to give her (or allow her to pick) a grandparental name.
As for what that is, exactly, methods vary. My parents chose grandparent names that they (and we) liked. My husband's parents didn't have a preference, so we gave started using names for them ("Grandma Mary" and "Grandpa Joe"). The kids themselves turned Grandma Mary into their own cute name. Grandpa Joe is still Grandpa Joe.