Although we only hear about Isabel and Grace indirectly, we think they're worth mentioning since they both illustrate one of the story's main themes: the strength of family.
Isabel is the narrator's wife. She is the one who makes Sonny and the narrator feel comfortable when Sonny moves back in with them; she displays a complete willingness to accept Sonny and welcome him back into the family. Isabel represents the kind of unconditional familial love that Sonny needs as he's trying to recover from his drug addiction.
Grace is the narrator's young daughter who dies, and her death from polio is symbolic of the horrible suffering we sometimes face. But her death is what prompts the narrator to get back in touch with Sonny, and in this way she's very much like her mother. Like Isabel, Grace helps the two men reconnect and she also illustrates the strength of family ties.