His sisters were his co-heirs and split the Barony of Kendal between them into "fees":
Helwise de Lancaster m. Peter de Brus II (Marquis Fee)
Alice de Lancaster m. William de Lyndesey (Richmond Fee)
Serota de Lancaster m. Alan de Multon (no children)
(There is a mistaken entry, on Peerage.com which says William was the father of Alice, his sister.)
From the calendars of the patent rolls (short-form references in parentheses):
He was appointed to custody over the county and castle of Lancaster in 1240 (HIII Vol.3 p.239) and the honour of Lancaster in 1234 (p.42) and 1241 (p.250).
He was given licence by the king to make a will in 1246 (p.480).
He died in November 1246. There is a detailed record of the inquest made into his deathbed enfeoffments.[2]
William spent some part of his young life as a hostage and prisoner for the part he and his father and he had played in the rebellion of the Barons against King John Lackland. On 22 Jan 1215/16 he and his knights, Ralph de Aencurt and Lambert de Bussay were taken at the castle of Rochester in arms against the king. From many records it is clear that as soon as his father passed away, although he was allowed to take up high positions, he was constantly being pressed by the king to pay debts which possibly went back to the damages of the rebellion.
He left enormous debts amongst England’s Jewry which passed along with many of the family’s rights to the heirs of his legitimate sisters. :Administration of these debts became a concern of the King himself. “1248 The debts of William de Lancastre due to the king, or to Aaron and Samuel, Jews of York, and David, Jew of Oxford, are granted to William of Valence, the king's [half] brother; Peter de Brus is ordered to pay off his part of the debts at the rate of £200 yearly; Cal. Pat. R., 1248, p. 33, cf. ib., 547.”[4]
Footnotes
↑ Chetham Society, (1898). Remains, Historical and Literary, Connected with the Palatine Counties of Lancaster and Chester. pp.307. Cheshire. Google Books.
↑ 'Kirkby in Kendale: c.1100-1350', in Records Relating To the Barony of Kendale: Volume 1, ed. William Farrer and John F Curwen (Kendal, 1923), pp. 1-22. British History Online http://www.british-history.ac.uk/kendale-barony/vol1/pp1-22 [accessed 12 July 2022].
Page 27: #114 William de Lancaster. "Peter de Brus(Bruce), of full age, and Walter son of William de Lyndeseya, age 16, are heirs."
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1. Alice who married William de Lindsay 2. Helewise who married Peter de
Brus.https://www.google.com/books/edition/The_Irish_landed_gentry_when_Cromwell_ca/Dp_HDwAAQBAJ?hl=en&gbpv=1&dq=william+iii+lancaster+agnes+de+brus&pg=PA96&printsec=frontcover