Infante Don Juan was born in Seville in 1264 as the eighth of the eleven children of Alfonso X of Castile and his queen Violante, daughter of King Jaime I 'el Conquistador' of Aragon. (Violante's sister Isabel married Philip III of France, so Juan and his siblings were first cousins of Philip IV as well as of Edward II.) Juan married twice: on 17 February 1281 to Margherita, daughter of Guillermo, marquis of Montferrat by his first wife Isabel de Clare, eldest sister of Edward II's brother-in-law Gilbert 'the Red', earl of Gloucester, and secondly on 10 January 1287 to Doña María Díaz de Haro, heiress of Biscay and daughter of the Don Lope Díaz de Haro whom Sancho IV murdered in 1288.
After Sancho IV's death, apparently of tuberculosis, in Toledo on 25 April 1295 - their cousin Edward of Caernarfon's eleventh birthday - Infante Don Juan was one of those who claimed the throne of Castile on the grounds that his nine-year-old nephew Fernando IV was illegitimate, and also, with the support of his ally the king of Granada, unsuccessfully attempted to seize the crown of Leon and the kingdom of Galicia. He temporarily made peace with Fernando in 1300, and a papal bull of 1301 declaring that Sancho IV's marriage to María de Molina had been legitimate also helped to calm the situation somewhat.  (Although as late as December 1306, it appears that the magnates of Castile, perhaps sick of all the in-fighting, wished to offer the Castilian throne to Edward of Caernarfon should Fernando IV die without a son.) This was not the end of Infante Don Juan's struggles with his nephew, however. When Fernando unsuccessfully besieged the Moorish-held town of Algeciras in 1309/10 - his son Alfonso XI finally took it in 1344 - Juan and his cousin Don Juan Manuel (from the last post) deserted him. A furious Fernando attempted to kill him in revenge and was only prevented from doing so by the intervention of his mother Queen María de Molina.  Juan was appointed one of the regents for his great-nephew Alfonso XI, Fernando IV's baby son, in 1312.
Beatriz, marchioness of Montferrat (1254-1286)
Infanta Doña Beatriz was the daughter of Alfonso X by his queen, Violante of Aragon (so was the sister of Infante Don Juan above and of King Sancho IV), and bore the same name as her illegitimate half-sister the queen of Portugal, from the last post. She was born in late 1254 as the second child of Alfonso and Violante; her sister Berenguela, who never married and founded the convent of Santa Clara de Toro, was a year older and was officially recognised as successor to the Castilian throne from May 1255 until the birth of their brother Fernando de la Cerda in October that year.
In 1271, the seventeen-year-old Beatriz married Guillermo, marquis of Montferrat, who had attended the wedding of her brother Fernando de la Cerda to Blanche of France two years earlier. Guillermo was born around 1240 as the son of Boniface II, marquis of Montferrat, and Margaret of Savoy; he was Edward I's second cousin via the Savoy connection (Edward's mother Eleanor of Provence was the daughter of Beatrice of Savoy). Guillermo's only child from his first marriage to Isabel de Clare, Margherita, married Doña Beatriz's younger brother Juan in 1281, in a double marriage alliance: their father Alfonso X of Castile was a very useful ally for Guillermo against his enemy Charles of Anjou, brother of Louis IX of France and king of Sicily from 1266.
5) Calendar of Close Rolls 1323-1327, pp. 175-176.
6) Close Rolls 1323-1327, pp. 253, 344, 350-351.