Sancha and Dulce of Leon were born the daughters of Alfonso IX of Leon by his first wife Theresa of Portugal. There were born in 1191 or 1192 and 1195 respectively. They had an older brother named Ferdinand, who died unmarried at the age of 22. Their mother Theresa had returned to Portugal in 1214 when her marriage to Alfonso was declared invalid because they were first cousins. She lived in the Monastery of Lorvão where she died in 1250. Their father remarried to Berengaria of Castile, but after having several children with her, this marriage was also annulled as Berengaria was his first cousin once removed.
Because of all this, a dispute arose as to who should inherit the throne. Alfonso’s son from his second marriage to Berengaria was Ferdinand III of Castile, who inherited Castile through his mother and with him securely in Castile Alfonso drew up a treaty with Portugal to allow his daughters from his first marriage to be his joint-heirs. He attempted to marry Sancha to John of Brienne, formerly the King of Jerusalem, but this was blocked by Berengaria of Castile. Instead, he married Sancha’s half-sister, another Berengaria.
This only seemed to strengthen Alfonso’s resolve, and he declared Sancha and Dulce his heirs, despite León never having had a female monarch before. Alfonso died on 24 September 1230, but the people of León refused to recognise his daughters as monarchs. Sancha and Dulce were most likely all too aware of this and not wanting to start a civil war the women, now 38 and 35 respectively, ceded their rights to their half-brother Ferdinand III in a treaty known as the Pact of the Mothers on 11 December 1230. They were richly rewarded with a yearly stipend and lordships. León and Castile were now united. Sancha and Dulce were de jure rulers of León for the brief period between Alfonso’s death and the signing of the treaty.
Sancha and Dulce retreated to the Monastery of San Guillermo in Villabuena, where Sancha died sometime around 1243, and Dulce died around five years later.
María Damián Yáñez Neira. 1982. “La princesa Doña Sancha, hija primogénita de Alfonso IX”. Tierras de León, 22(47):47–60. online (Spanish!)