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Isobel married Robert DE BRUS 'the competitor', 5th Lord of Annandale, son of Robert DE BRUS 'the noble', 4th Lord of Annandale and Isabella of Huntingdon, by 1240.1 (Robert DE BRUS 'the competitor', 5th Lord of Annandale was born about 1210 and died by 3 May 1294 1.)
He (Robert, 5th of Annandale) married, first, in or before 1240, Isobel, second daughter of Gilbert de Clare, third Earl of Gloucester and Hereford. She was born 1226, and was only thirteen when married ...
e-books, The Scots Peerage ed. Sir James Balfour Paul vol. 2 (1905).