[1] Isabella was born in Toro, the eldest daughter of King Sancho IV of Castile[2] and María de Molina.

As On 1 December 1291, Isabella married King James II of Aragon in the city of Soria.

James chose to change his alliances and take advantage of the turmoil inside Castile.

He had their wedding annulled and proceeded to marry Blanche of Anjou.

In 1310, at Burgos, she married her second husband, Duke John III of Brittany.