Larry Mackleberry is the twin brother of Jerry Mackleberry and the brother to Sherri, Terri and Unnamed. He and Jerry attended Sherri and Terri's funeral in "Treehouse of Horror XXVII".
Both Larry and his brother Jerry's existence is non-canon, as their only appearance was in the above-mentioned Treehouse of Horror episode, which are considered non-canon to the regular shows' continuity.