James I died on 27 July 1276 and Peter succeeded to the throne. Constance became his Queen consort. With the War of the Sicilian Vespers (1282–1302), at first Peter and then their sons claimed the throne of Sicily in her right. The War resulted in the partition of the Kingdom of Sicilyto the "Kingdom of Trinacria" (the island of Sicily) under her descendants and the Kingdom of Naples under the descendants of Charles of Anjou.
Peter III died on 2 November 1285 and Constantia remained a widow. She ended her days as a nun in Barcelona.