The Oxford DNB biography of Robert de Ros states that Robert "was the son of Everard de Ros (d. 1182/3) and Roese Trussebut (d. 1194). He had one brother named Peter. From his father Robert inherited Roos in Holderness, from which his family received its name, and the lands the Ros family had inherited through marriage from Walter Espec, centring on Helmsley in Yorkshire and Wark in Northumberland. Robert de Ros succeeded to his father's lands in 1191, paying a relief of 1000 marks, although he may not have gained control of Wark until 1200. From his mother he inherited one-third of the Trussebut lands and the hereditary office of bailiff and castellan of Bonneville-sur-Touques in Lower Normandy, near which the Norman lands of the family lay." [1]
[1] W. E. Rhodes, rev. Hugh M. Thomas, "Ros, Robert de (c. 1182–1226/7), Baron" Oxford Dictionary of National Biography (Oxford University Press, 2004, online edition), [Oxford_Dictionary_National_Biography], [OxfordDNB(UM)].