Welcome to the site, Hesadevil!
The title 'granny' is an honorific. It's a bit like the way that all the women neighbours when I was growing up were known as 'auntie' even though they weren't related to me.
You'll notice however, that in Equel Rites, everyone calls her Granny whereas in the later ones, it's only certain people who are allowed to call her that (Tiffany for ewxample). Everyone else calls her Misstress Weatherwax - at least to her face.
“Men never commit evil so fully and joyfully as when they do it for religious convictions.” – Blaise Pascal