Her grandfather, Sir John II of Brienne, was the third son of John of Brienne, King of Jerusalem and Emperor of Constantinople, and his third wife Berengaria of León, which made Margaret a cousin of Queen Eleanor of Castile.
Her brother, Jean de Fiennes (d. 1340), married Isabel, daughter of Guy de Dampierre, Count of Flanders and his second wife Isabel of Luxembourg.
He had succeeded to his father's lands and barony in 1282 and was already a national hero after killing Llywelyn ap Gruffudd, his cousin, in battle.
Her husband died on 17 July 1304 and she lived until 1333, probably being buried in Wigmore Abbey.
This biography of a baron or baroness in the Peerage of England is a stub.