Isabella's paternal grandparents were Richard de Clare, 3rd Earl of Hertford and Amice FitzRobert.
Isabella was the fourth of six children, her brother was Richard de Clare, 6th Earl of Hertford.
Isabella was married on 12 May 1240 (at age thirteen and a half) to Robert de Brus, 5th Lord of Annandale.
[citation needed] Robert and Isabella had up to six children: John Balliol's time as King of Scotland did not last long, he died in 1314.
Her husband married a second time, to Christina de Ireby, this marriage produced no children.