About Isabella Randolph
Isabella Randolph was the daughter of Sir Thomas Randolph, 1st Earl of Moray, and his wife Isabella Stewart of Bonkyll. Some texts show her name as Geilis. She was born c.1320 and died c.1395. She married John de Dunbar of Derchester and Birkynside. She was the mother of George de Dunbar I, Earl of March.
Of note, Isabella's son George was recognized as both cousin and uncle to her sister Agnes' family. From Piscottie, the Scottish chronicler, author of The Historie and Chronicles of Scotland, 1436–1565, (the first history of Scotland to be composed in Scots rather than Latin), we learn that:
George was a son of John de Dunbar of Derchester & Birkynside, by his spouse Geiles (or Isabella), daughter of Thomas Randolph, 1st Earl of Moray (d. 1332). John was son of an Alexander de Dunbar, Knt. Alexander was a younger brother of Sir Patrick de Dunbar, 9th Earl of March. George succeeded his cousin or uncle Sir Patrick in his honours and estates, and appears in a charter dated 28 June 1363; and is second witness, styled 'cousin' of Sir Patrick and his wife 'black' Agnes, in another charter signed at Dunbar Castle on 24 May 1367. "Robetus de Lawedre, consanguineus noster" witnessed a charter of "Georgii comitis Marchie" relating to Sorrowlessfield, in the reign of Robert III (1390–1406), indicating both his extended family and that he was active in the management of the Dunbar family estates during Robert's reign.
In brief, Sir Patrick de Dunbar was the husband of Isabella' sister Agnes. Sir Patrick had a younger brother Alexander. Isabella married Alexander's son, John de Dunbar. When Patrick died, George inherited the earldom. George married a Seton and his daughter Janet married a Seton. His son George II married (1) Beatrix and (2) Hawise de Hay, who were said to be closely related.