In 1480 he had sasine to Cousland and Dysart. On 17th June 1486, Nicholas Ramsay was confirmed in Wilston and Carberry, which William Sinclare resigned; and on 20th July 1487, there is confirmation of a sale by William de St.Clair, Baron de Newburgh, with consent of Christian Leslie his spouse, of the lands of Archadlie in Newburgh with the signature of William Lord Synclare, at Stirling.
Peerage writers inform us that his life was spent in a struggle with the more favoured brothers for a share of the paternal inheritance.
William, Earl of Orkney and Caithness, having also infeft Sir Oliver of Roslyn, his son of a second marriage, in all his baronies lying within the sheriffdoms of Edinburgh, Fife, and Stirling, to the prejudice of William Sinclair of Newburgh, his eldest son of the first marriage: It was sustained by Newburgh, post excessum patris, that the disposition made by his deceased father was null, (1) in respect he was the only child that had undoubted title to those lands by the common law, as being heir to the deceased Earl, and so could not be prejudged by any private deed made in behalf of the younger children, since the collector of our Regiam Majestatem, lib. 2, cap. 27, art. 2 de successione filii ad patrem, expresses himself thus: Si pater fuerit miles, tunc eo casu, ejus filius primogenitus succedi tin totum ita quod nullus fratrum suorum partem inde, de jure, petere potest. (2) That in the greatest rigour, and even stretching the law, his father was only allowed by the 2nd chap. art. 2 to give filiis postnatis, id est, post primogenitum natis, partem rationabilem de haereditate sua, cum consensu haeredis: Whereas he had granted to Sir Oliver the bulk of his estate lying as said is in Fife, and towards the South of Forth, without his consent: and by that means he had deprived him of his right to the succession: Which was his third exception. Which right could no more have been abstracted from the heir than the roundness from a bowl or sphere; according to the 20th chap. art. I, Non licet filium ex haeredare.
After some replies and duplies made by the advocates, the lands in debate were divided by the agreement of both parties, under form of instrument, whereby Sir Oliver and his heirs resigned and gave over to William Lord Saintclair of Newburgh, his elder brother and his heirs all the lands of Cousland, with their pertinents, lying within the sheriffdom of Edinburgh; and his lands of Dysart within the barony of Samin; with lands of Ravenscraig and castle of same; the lands of Dubbo, Carberry and Wilston, lying in Fife; patronages of kirks and chaplainries, office of bailery, etc., to remain heritably with the said William and his heirs perpetually [and shall give William all charters, sure evidents, infeftments, and obligations that he has, or may give of the said lands, or of any lands that may be profitable to the said William and not skaithless to himself in other lands, nor to his younger brother William].
And on the other hand, William of Newburgh, designated "Eldest son and heir to the Magnificent and Potent Lord, William Earl of Orkney and Caithness"; and his son Henry likewise called "Eldest son and heir apparent to said William, Lord of Newbrugh"; renounce their interest to the baronies of Roslin, the castle of the same, and patronage of the college, or provostry, to the lands of Pentland, Pentlandmoor, Morton and Mortonhall, and to the barony of Herbertshire.
And both of them bind themselves to stand by one another, under the penalty of £5,000, whereof £3,000 were payable to the King, and £2,000 to the Archbishop of St.Andrew's nomine poenae within 40 days; as also under the penalty of infamy and inhability, and of being mensworn men, they having sworn on the Holy Evangels the performance of the foresaid articles.
And it is thereby declared that Sir Oliver shall worship and honour the said William as effeirs, and accords him to do to his eldest brother. And if there happen any plea or contestation betwixt the said William of Newburgh and William his younger brother for the Earldom of Caithness, the said Sir Oliver shall stand neuter between them, as he should do betwixt his brethren, and take no part with either of them during the quarrel.
The agreement is dated 9th February 1481. Witnesses: William, Archbishop of St.Andrew's; Andrew Stuart, Lord Avondale, Chancellor Scotland; Colin, Earl of Argyll; John, Earl of Atholl; William, Bishop of Ross; Andrew Stuart, Provost of Lincluden: Mr. George Carmichael, Treasurer of Glasgow; Alexander Lumsden, rector of Flisk; and Alexander Borthwick, clerk of St.Andrew's. And sealed with the seals of the Archbishop of St.Andrew's, the Lord Chancellor, and the Earl of Argyll, and subscribed by the public notaries. This agreement is ratified 18th February 1481. [Scottish Antiquities]
NOTE - Surely this agreement between Lord St.Clair and Sir Oliver is still extant.
William of Newburgh, third Lord Sinclair, is generally called "William the Waster"
He died in 1487, and was interred at Dunfermline, leaving by his wife Lady Christian Leslie, daughter of George, first Earl of Rothes -