Corrected entry: When Grandma / Esther takes Jon-boy aside to ask him about his grandfather's whereabouts, she says "Zeb" instead 'your Grandpa'.
Correction: While the kids are entertaining themselves with hand shadow puppets, Esther pulls John, not John-Boy, aside and asks, "Isn't Zeb with you?" In fact, twice she says "Zeb" to John, about his own father. And later while Zeb is sitting on Ike's pool table, Zeb says, "Esther" when he's talking to John-Boy. Zeb does this again with John-Boy when he learns Esther's going to the dance. This type of thing occurs throughout the series; it's normal.
Correction: That's not a mistake, character or otherwise. She simply used her husband's name. She even says Zeb several times through the episode. My own Grandparents and parents have switched between using their spouses' names or saying "your ___" depending on the situation.
Bishop73