Herleva - William I's Mother

If William the Conqueror had a half sister (Beatrix, supposed wife of Alphonsus de Vere) she would not be a child by Herleva. Robert I, Duke of Normandy,

Herleva[a] (c.1005 – c.1050) was an 11th-century Norman woman known for having been the mother of William the Conqueror, born to an extramarital relationship with Robert I, Duke of Normandy, and also of William's prominent half-brothers Odo of Bayeux and Robert, Count of Mortain, born to Herleva's marriage to Herluin de Conteville.