 Her only surviving brother was Thomas Fitzalan, 12th Earl of Arundel, of whom Joan was his co-heiress.She had an older sister Lady Elizabeth FitzAlan who married as her second husband Thomas Mowbray, 1st Duke of Norfolk.Her paternal grandparents were Richard Fitzalan, 10th Earl of Arundel and Eleanor of Lancaster, and her maternal grandparents were William de Bohun, 1st Earl of Northampton and Elizabeth de Badlesmere.