Beatrice's paternal grandparents were Ferdinand III of Castile and Elisabeth of Hohenstaufen, herself daughter of Philip of Swabia and Irene Angelina.
Beatrice was originally betrothed to John II, Margrave of Brandenburg.
Her betrothal was arranged by her father to attract the support of Brandenburg for his candidacy for the throne of Germany.
[1] Although the name of the king's daughter is not specified in the source, it could only have been Beatrice as her older sister Berenguela was already betrothed at that date.
Beatrice married in Murcia in August 1271 to William VII, Marquess of Montferrat.