Complex sentence: She immediately became heiress presumptive to the throne of Castile upon her birth and was formally recognised as such and as Princess of Asturias on 1 January 1423 by the Cortes of Toledo.
Split into simpler sentences: She immediately became heiress presumptive to the throne of Castile upon her birth. The Infanta was formally recognised as successor to the throne of the kingdom and sworn in as Princess of Asturias on 1 January 1423 by the Cortes of Toledo.