uncle-in-law

Between uncle and aunt-in-law, or uncle's wife, you have a connection of affinity, and you have the same between aunt and uncle-in-law, or aunt's wife. [sic]

Vernon would later become an uncle-in-law to Harry's wife, Ginny and a great-uncle to Harry and Ginny's children, James Sirius Potter, Albus Severus Potter and Lily Luna Potter.