Cousin Francine a.k.a Mother Shabubu (formerly Frank) is Homer's cousin.
She was mentioned In the episode "Lisa's First Word", while discussing the possibility of moving from their small apartment due to lack of space because of the arrival of Lisa, Homer says that Lisa can have Bart's room, and Bart will sleep with them until he is 21. Marge asks "Wouldn't that warp him?" And Homer replies, "My cousin Frank did it, and he was fine." To which Marge replies, "You don't have a cousin Frank!" Homer continues to explain that, "He became Francine back in '76, then he joined that cult. I think his name is 'Mother Shabubu' now."
It's possible that her last name isn't Simpson, and that she is his cousin on his mother's side.