I am unsure of that episode, but in the show Fester is Morticia’s Uncle. However the wiki says he is Granny Frump’s oldest son, making him Morticia’s Brother. Which is what I always thought he was until the movie.

She is Morticia’s mother.

Edit: part of the confusion is there is Granny Frump, and Grandmama. Grandmama being Gomez’s mother.

Grandmama, has a wart on her nose… This is Grandmama.

I first shared her back in week 2 i think (slrpnk is down currently and my links are not working), Morticia asks to borrow a cup of cyanide from her.