The mother of the Riddles' mom is pictured at the start of I Am Tomioka Tessai when a flashback is shown of her making a train with her as a child.
She is the maternal grandmother of Yadina and Xavier, which means her own mother would've been one of their "great grandmothers", and whose own mother could've possibly been Great-Great Grandma Riddle, meaning that the Riddles' grandmother could've been the grandmother of GGG Riddle.