The Northumbrian royal descent of Hereswith is traced from Edwin of Northumbria (who ruled from about 616 to 632), the son of Ælle, King of the Northumbrian kingdom of Deira. Hereswith was the daughter of Edwin's nephew, Hereric. After Ælle's death, his heir Edwin was sent into exile by the rulers of the northern Northumbrian kingdom of Bernicia. He had at least two siblings, including a sister named Acha, and was Hereswith's grandfather on her father's side. Edwin was received in exile at the court of the powerful Welsh ruler Cadfan ap Iago of Gwynedd and in childhood he was the companion of Cadfan's son Cadwallon.