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Since the Breton War of Succession, Brittany had been understood to operate according to the semi-Salic Law: women could only inherit if the male line had died out. As expected from the provision of the Treaty of Guérande, which ended the war, neither Margaret nor her younger sister Marie were recognized as heirs to the duchy. After her father's death, her uncle Peter II of Brittany succeeded as Duke of Brittany. He was also childless, and to avoid any subsequent dispute, he arranged the marriage of Margaret to his first cousin, Francis of Étampes, the second in the order of succession and the last male left of the Breton House of Montfort; also, he arranged the marriage of the younger sister Marie to John II, Viscount of Rohan, the most powerful noble in Brittany.
Marriage
On 13 or 16 November 1455, Margaret was married to Francis of Étampes, at the Château de l'Hermine in Vannes. She became Duchess of Brittany upon his accession in 1458. Their only child John, Count of Montfort (29 June – 25 August 1463) died in infancy.
Margaret inherited The Hours of Isabella Stuart from her mother, and a depiction of Margaret kneeling before the Virgin and Child at a prayer desk, draped in fabric embroidered with the arms of Brittany, was added to the manuscript around the time of her marriage. It is now held in the collection at the FitzWilliam Museum.