And let’s just put this issue out there - sometimes even the word “grandmother” itself is used to refer to a non-specific female ancestor that is at least 2 generations removed. Have you ever heard someone refer to a great (or greater) grandmother as just “a grandmother”? I know I have. And that isn’t incorrect either; see the second definition of “grandmother” in M-W’s dictionary:
Definition of grandmother
1 : the mother of one's father or mother
2 : a female ancestor
https://www.merriam-webster.com/dictionary/grandmother