Kate Pewterschmidt is the sister of Lois Griffin, to whom she bears a striking resemblance. She's a horrid, feral woman, being raised in a reinforced steel cage, hidden away in Carter's basement, where she's occasionally treated, just enough to keep her alive.
Kate is referenced to at the end of the animatics part of the "Exclusive Online Content" in the Family Guy Volume 4 DVD set for "The Fat Guy Strangler". In the clip, Barbara Pewterschmidt asks Carter if Lois knows about "Kate" to which Carter quickly replies that no one knows about Kate. The scene cuts to Carter throwing a piece of meat into a basement next to a small cage. It holds what looks like a disfigured and savage form of Lois. She quickly snatches up the meat and eats it.