Sabo's yet unnamed mother is a noble of the Goa Kingdom. She and her unnamed husband adopted Stelly as a backup heir and son. She has a stereotypical personality of a Noble, and only seeks to further their noble status. This was clearly seen when her son got into a fight with a noble boy of a higher status who used a weapon on him, she apologized to the family while comforting the boy, completely ignoring her own son's injuries.

(Source: One Piece Wikia)