Research Notes:
Complete Peerage states, [1]
William (de Ferrieres), Earl of Derby, son and heir of Robert, succeeded in the 1150s. He was one of the adherents of the younger Henry on his rebellion in Apr. 1173 and sacked and burned Nottingham in May or June 1774. He made his submission to the King at Northampton, 31 July 1174, surrendering the castles of Tutbury and Duffield. The king took him with other prisoners, to France in Aug. following, and imprisoned them at Caen. He m. Sibyl, da. of William de Braiose, Lord of Bramber, by Bertha (heiress of Brecon and Over Qwent), sister and coh. of William de Hereford, and 2nd da. of Miles (de Gloucester), Earl of Hereford. He d. on Crusade, at the siege of Acre in Palestine, in 1190, before 21 Oct. His wife survived him and was perhaps living as late as 5 Feb. 1227/8.
Keats-Rohan states of Sibilla de Braose that she was the "Daughter of William II de Braose (d. 1192/93)" and the "wife of William earl Ferrers (d. 1190)." [2]
Keats-Rohan states of Willelm de Ferrariis that he was the [3]
Son of Robert II earl of Ferrers, whom he had succeeded by 1160. He married Sibil, daughter of William II de Braose (d. 1192). He died in 1190, leaving issue William II, (d. 1247), Hugh, Thomas, and pernel, wife of hervey II Bagod of Stafford. Sanders, 149, Comp. Peer. iv, pp. 190-203.
[1] George E. Cokayne and Vicary Gibbs, ed., The Complete Peerage, rev., Vol. 4, Dacre to Dysart (London, St. Catherine Press, 1916), 192-194, [InternetArchive].
[2] K. S. B. Keats-Rohan, Domesday Descendants, A Prosopography of persons occurring in English documents, 1066-1166: II Pipe Rolls to Cartae Baronum (Boydell Press, 2002), 346.
[3] K. S. B. Keats-Rohan, Domesday Descendants, A Prosopography of persons occurring in English documents, 1066-1166: II Pipe Rolls to Cartae Baronum (Boydell Press, 2002), 460.