As a consequence of his visit to Santiago de Compostela, Alfonso IX invited him to marry his daughter Sancha and, presumably, through her inherit the Leonese throne.
However Berengaria of Castile, a long time divorced and an inheritor in her own right of the Castilian throne, main advisor of her son Ferdinand III, offered one of her own daughters to John instead.
By agreement, 12-year-old Baldwin had been betrothed to the 5-year-old Marie of Brienne, the daughter of John and Berengaria, since 19 April 1229 to firmly establish the dynastic alliance of the two co-emperors.
The "Obituaires de Sens Tome" of the Abbey of Maubuisson record that Berengaria died on 12 April 1237, aged around 33, surviving her husband by only sixteen days.
Other books, however, confuse her coffin as being that of Infanta Berengaria (1228–1288), a nun at the Cistercian monastery Santa María la Real at Las Huelgas.