What do you call your godmother's mother?

Answer 1

Your half-sister's mother may be your mother, if you have different fathers.

If you have different mothers and your father is married to your half-sister's mother, then you probably should call her "mother."

If your half-sister's mother is not connected to your father (i.e. her father married your mother) then there is no relationship name for you to call her mother, other than "my half-sister's mother."

Answer 2

Well you would call your mother's mother grand mother so I would assume that you would call your godmother's mother grand godmother.