"Uncle" Ike McCaslin is not anyone's actual uncle. In his first appearances ("Lion" is the second of these) he is one of the men who are part of De Spain's annual hunting parties, and the title "Uncle" seems to represent a form of deference to his age and to his skill as a woodsman. He becomes a major character in the novel Go Down, Moses (1942), and there he is explicitly referred to as childless; in this story, however, where he plays a minor role, he has a grandson.
