She is married to Walter Stewart.
They got married before 1361.Source 1
Duncan, Earl of Fife, by his wife Mary de Monthermer
Isabella MacDuff, Countess of Fife (c. 1320-1389) was a Scottish noblewoman who was Countess of Fife from 1363 until she resigned the title in 1371. She was the only child of Duncan, Earl of Fife, by his wife Mary de Monthermer, daughter of Ralph, Lord Monthermer and Joan of Acre.
Scotland in the fourteenth century had no shortages of heiresses. The great earldoms of the country, frequently in some cases, passed through and into the hands of dowagers and heiresses at various points in the late Middle Ages. Across Europe events such as plague or ongoing warfare had an adverse effect on the male population, and so, noblewomen and their position in society, law, and politics became an increasingly urgent question. Isabella of Fife's position as a woman with significant land, wealth and potential influence was not a unique one.[1]
In 1332 she and her mother had been captured at Perth by supporters of Edward Balliol. She was sent as a ward to Northumberland.
Her first husband was Sir William Felton of Northumberland (died c. 1358) in Fife, whom she married around 1358. He died soon afterwards, leaving her with three children. She was next married, before June 1361, to Walter Stewart, second son of Robert Stewart, later King Robert II. He died without issue the following year, and she was married again in January 1363 to Sir Thomas Bisset of Upsetlington in Berwickshire. He had died by April 1366. She was married for a fourth and final time to John Dunbar, who died before 1371. She had three children by her first marriage.
Isabel was persuaded to resign the earldom on 30 March 1371 to Robert Stewart, Earl of Menteith (later Duke of Albany), who was her brother-in-law by her second marriage. She died shortly after 12 August 1389 and was buried next to Walter Stewart.
Robert II King of Scotland
Scottish Monarch. Son of Walter, Steward of Scotland and Marjory Bruce, daughter of Robert the Bruce. He served as Regent for David II twice, while David was in exile in France and again while imprisoned in England. Robert succeeded David in 1371, and was crowned at Scone on March 26. His first marriage to Elizabeth Mure and the 9 children of the union were declared illegitimate, as the couple were too closely related. A Papal dispensation was acquired in 1347, but to many it still wasn't enough. Upon the death of his first wife he married Euphemia Ross, who would become his queen, in 1355. They had four children. The question of the legitimacy of his first marriage would later play a big part in the succession. Robert is thought to have had 21 or more children in total, including at least 8 illegitimate children by various mistresses. It was said about Robert that "A more tender heart no man could have". Most of his 19-year reign was troubled by wars he could play little part in. He allowed his son Robert, Earl of Carrick (later Robert III), to act in his stead most of the time. Old and infirm, he died at Dundonald Castle at the age of 74.
Fascinating and authoritative of Britain's royal families from Henry VIII to Elizabeth I to Queen Victoria, by leading popular historian Alison Weir
'George III is alleged to have married secretly, on 17th April, 1759, a Quakeress called Hannah Lightfoot. If George III did make such a marriage...then his subsequent marriage to Queen Charlotte was bigamous, and every monarch of Britain since has been a usurper, the rightful heirs of George III being his children by Hannah Lightfoot...'
Britain's Royal Families provides in one volume, complete genealogical details of all members of the royal houses of England, Scotland and Great Britain - from 800AD to the present. Drawing on countless authorities, both ancient and modern, Alison Weir explores the crown and royal family tree in unprecedented depth and provides a comprehensive guide to the heritage of today's royal family - with fascinating insight and often scandalous secrets.