John Sigismund was recognized as independent prince of Transylvania and of sixteen adjacent Hungarian counties, Queen Isabella to act as regent during his minority.
He was mixed up with the sordid intrigues which preceded the deposition of Edward II., and supplied Queen Isabella and Mortimer in Paris with money in 1325 from the revenues of Guienne, of which province he was treasurer.
The other children were Louise, consort of Leopold I., king of the Belgians; Marie, who married Prince Alexander of Wurttemberg and died in 1839; Louis Charles, duc de Nemours; Clementine, married to the duke of Coburg-Kohary; Francois Ferdinand, prince de Joinville; Henri Eugene, duc d'Aumale; Antoine Philippe, duc de Montpensier, who married the Infanta, younger sister of Queen Isabella of Spain.
These plans were artfully fostered by the Savoyard kinsmen of Eleanor, daughter of Raymond Berenger, count of Provence, whom he married at Canterbury in January 1236, and by his half-brothers, the sons of Queen Isabella and Hugo, count of la Marche.
William de Wykham, as he is called in earlier, William Wykeham in later life, has been variously guessed to be the son of a freedman carpenter, and an illegitimate son of Queen Isabella and Roger Mortimer (Notes and Queries, 10th s.
When Queen Isabella and her husband were forced to leave Spain by the revolution of 1868 he accompanied them to Paris, and from thence he was sent to the Theresianum at Vienna to continue his studies.
The king's conduct, however, drew him to the side of the earl, and he had already joined Edward's enemies when, in October 1321, his wife, Margaret de Clare, refused to admit Queen Isabella to her husband's castle at Leeds in Kent.
At last, in August 1839, exhaustion brought the Basques to reccgnize the government of Queen Isabella by the convention of Vergara in return for the confirmation of their privileges.
When Queen Isabella took up arms against her husband in 1326 she was joined at once by the earl, who took a leading part in the proceedings against the king and his favourites, the Despensers, being Edward's gaoler at Kenilworth castle.