Sibil m. 1st, John de Vipont; 2ndly, Franco de Mohun. [Sir Bernard Burke, Dormant, Abeyant, Forfeited, and Extinct Peerages, Burke's Peerage, Ltd., London, 1883, p. 197, Ferrers, Earls of Derby]
NOTE: Brian Tompsett, Directory of Royal Genealogical Data, University of Hull, UK, has her only as daughter de Ferrers, daughter of William de Ferrers, 4th Earl of Derby, and Sibyl de Braose.