Big Ma is the Logan children's grandmother, but she's not sitting around doing needlepoint portraits of her Chihuahuas. She's tall and strong, and her hands are big and rough from all her hard work (2.1). She works in the fields like a younger woman (1.18), does all the housework, and dispenses medical advice to people from all over the area (1.30).
We kind of wish someone would give this woman a rocking chair, because we get the feeling she's worked hard enough. Her own parents were slaves, and she had six children, but only two are still alive (Papa and Uncle Hammer). But she's learned from her hard life: she's the lore-keeper and storyteller of the family, and she dispenses most of what we learn about the Logan family's past.