[3] He was son of Henry I Sinclair, Earl of Orkney, by his wife Jean, daughter of John Halyburton of Dirleton.

He married Egida Douglas, daughter of Sir William Douglas of Nithsdale and Egidia Stewart, and the maternal granddaughter of King Robert II of Scotland.

Sir William Douglas was murdered by a group of assassins who had been employed by Lord Clifford and as a result Sinclair inherited through his wife the whole of the Lordship of Nithsdale.

[2] Sinclair was one of those captured following the Battle of Homildon Hill in 1402, but released on ransom.

[4] In about 1407 he married Egidia Douglas, daughter of Sir William Douglas of Nithsdale and wife Egidia and maternal granddaughter of King Robert II of Scotland and Euphemia de Ross.