The Riddles' dad's grandma is the maternal or paternal grandmother of the Riddles' dad (father of Xavier and Yadina).
She is the great grandmother of Xavier and Yadina: Yadina refers to her as "great grandma" and she is shown in a flashback at the end of I Am Tomioka Tessai.
Yadina recalls "she made the best mushroom empanadas" and was shown beside her, indicating she had known her.
Empanadas are a food originating in Spain and common in Latin American and Filipino cultures.
Given the pale skin tone of the Riddles' mom, it is likely that this is a great grandmother on Yadina's dad's side, who has a darker skin tone.
The in-law[]
Since Great-Great Grandma Riddle was only depicted having a single child (a son, the "great grandpa") it isn't initially clear whether Great-Great Grandma Riddle's son (Greatx1 Grandpa Riddle, not to be confused with his dad Greatx2 who resembles Brad) continued the family line via a son or a daughter.
Initial assumptions that Yadina's great-grandma was the daughter "Great Great Grandma" are contradicted by her not being shown as a little girl with the son: more likely he is the woman she married, making her the daughter-in-law of great-great grandma Riddle instead.
It is clear that both are ancestors of the Riddles' dad due to their dark skin (the Riddles' mom has pale skin)
Since both of the Riddles' dad's parents (Riddles' dad's dad and Riddles' dad's mom) have dark skin, EITHER of them (the paternal grandparents of Xavier and Yadina) could be the child of Great-Great Grandma Riddles' son and his wife (the Great Grandma that Yadina met before her likely death)