John Ravilious has Sybil as sister of Walter de Mandeville (who John'ssource says died without issue). I have Sybil as daughter of a Walter(who I think was father of both Sybil & Walter). Walter, the father, isnot in John Ravilious' pedigree, but the dates would be stretched a bit,I think, without the extra generation.
(Source: Family histories and genealogies : a series of genealogical andbiographical monographs on the families of MacCurdy, Mitchell, Lord,Lynde, Digby, Newdigate, Hoo, Willoughby, Griswold, Wolcott, Pitkin,Ogden, Johnson, Diodati, Lee and Marvin : and notes on the families ofBuchanan, Parmelee, Boardman, Lay, Locke, Cole, DeWolf, Drake, Bond andSwayne, Dunbar and Clarke, and a notice of Chief Justice Morrison RemickWaite : with twenty-nine pedigree-charts and two charts of combineddescents
unknown: Priv. print., 1892, page 260 of 1,893 pgs. )