Annales of Alfred of Beverley is in Late Medieval Books.
[4th September 925] And at Kingston he was consecrated king by Athelm, archbishop of Canterbury. In the second year of his reign he gave his sister in marriage, with great solemnity, to Sihtric, king of the Northumbrians, of Danish stock. When Sihtric died a year later, Æthelstan, having driven out Guthfrith, the son of Sihtric, added the kingdom of the Northumbrians to the kingdom of the West Saxons.
Et in Kingestuna ab Athalino Dorobernensi archiepiscopo consecratur in regem. Qui anno regni sui secundo sororem suam Nordanhymbrorum regi Sitrico Danica stirpe cum magna [sollemnitate] in matrimonium dedit. Sitrico post annum mortuo, Athelstanus, expulso Guthfritho Sitrico filio, regnum Nordanhymbrorum regno adjecit Westsaxonico.