Another name for Isabella was SIBBALD, Isobel.3
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Isabella married George DOUGLAS 4th Earl of Angus, 'Red Douglas', son of William DOUGLAS 2nd Earl of Angus and Margaret HAY, before 1445.1 2 (George DOUGLAS 4th Earl of Angus, 'Red Douglas' was born about 1420 and died on 12 March 1463 2.)
"George, fourth Earl of Angus, married Isabella, only daughter of Sir John Sibbald of Balgony, in Fife, Master of the Household to King James II. The date of the marriage has not been ascertained, but Godscroft asserts it took place in the lifetime of James, third Earl, or before 1445. She survived the Earl for many years, but was dead before February 1503. She seems to have married twice after the Earl's death her second husband being a person named Carmichael, whom Godscroft says was a younger brother of Carmichael of that Ilk, though he discredits the story for want of evidence, but a James Carmichael and Peter Carmichael of Dron are named as brothers of the fifth Earl of Angus, and Peter Carmichael appears as executor or intromitter with the estate of Elizabeth or Isabella, Countess of Angus, in February 1503, after her death. The Countess's third husband was Sir Robert Douglas of Lochleven, and after 1489 he is frequently named along with her as a litigant before the Lords of Council. By his wife, George, fourth Earl of Angus, had issue ..."
Isabella next married John CARMICHAEL of Balmedie, son of John CARMICHAEL of Meadowflat, Constable of St Andrews and Unknown.1 3 (John CARMICHAEL of Balmedie was born about 1425 and died before 20 February 1489 3.)
"John of Balmedie appears as 'Johannes Carmychell de Balmathe,' 20 December 1477, witness to a charter of Archibald, fifth Earl of Angus. He acquired the lands of Balmedie by his marriage, about 1477, with Isobel, daughter of Sir John Sibbald of Balgonie, and widow of George, fourth Earl of Angus. He was dead before 20 February 1489, when she was wife of Sir Robert Douglas of Lochleven, having had by her, who was dead before 21 February 1497"
Isabella next married Sir Robert DOUGLAS of Lochleven, son of Sir Henry DOUGLAS of Lochleven and Lugton and Elizabeth ERSKINE, after 1479.1 (Sir Robert DOUGLAS of Lochleven was born about 1438 and died on 9 September 1513 in Battle of Flodden Field, Northumberland, England 1.)
"Robert Douglas of Lochleven ... married, secondly, after 1479, Isobel, only daughter of Sir John (not Andrew, as in Douglas) Sibbald of Balgony, relict of George Douglas, fourth Earl of Angus, and of John Carmichael of Balmadie..... By his first wife only he had issue."
1 e-books, The Scots Peerage ed. Sir James Balfour Paul vol. 6 (1909).
2 e-books, The Scots Peerage ed. Sir James Balfour Paul vol. 1 (1904).
e-books, The Scots Peerage ed. Sir James Balfour Paul vol. 4 (1907).