Keats-Rohan states, [1]
Robert Corbet
Norman, brother of Roger (q.v.), tenant of Earl Roger in Shropshire, successors of their father Corbet (Ord. Vit. ii, 272). His daughter Sibil, mistress of Henry I and mother of Rainald earl of Cornwall, married Herbert fitz Herbert the chamberlain, and her sister Alice married William I Boterel of Cornwall. L. Musset, 'Administration et Justice', Cahiers des annales de Normandie no. 17 (1985), 147, derives the family from the region of Boitron and Essay, near Sées, Orne.
[1] K. S. B. Keats-Rohan, Domesday People, A Prosopography of persons occurring in English documents, 1066-1166 (Boydell & Brewer Ltd, 1999), 373, [GoogleBooks].