 In November 1235, Blanche of Castile's nephew, King Ferdinand III of Castile, lost his wife, Elisabeth of Hohenstaufen, and Blanche's sister Berengaria of Castile, Ferdinand's mother, was concerned that her widowed son might involve himself in liaisons that were unsuited to his dignity as king.Berengaria determined to find Ferdinand another wife, and her sister Blanche suggested Joan of Dammartin, whose marriage to the king of Castile would keep her inheritance from falling into hostile hands.In October 1237, at the age of about seventeen, Joan and Ferdinand were married in Burgos.