aunt-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]

Petunia would later become an aunt-in-law to Harry's wife Ginevra Weasley and a great-aunt to Harry and Ginny's three children; James Sirius, Albus Severus,