Chester John "Chet" Simpson, also known as Great-Uncle Chet, is Bart, Lisa, and Maggie's paternal granduncle who runs an unsuccessful shrimp company. Since he inherited the Simpson Gene, he must be Abe's brother, Marge's uncle-in-law, and Homer's paternal uncle. Judging from his appearance, he is most likely the youngest of Abe's brothers.
He may have died off-screen at some point because in Mathlete's Feat, Grampa mentions having 11 dead siblings.