 His father was John III of Sweden and his mother was Gunilla Bielke.John's half-brother was King Sigismund III of Poland (1566–1632, reigned in Sweden in 1592–99, and in the Polish–Lithuanian Commonwealth in 1587–1632).His uncle was Charles IX of Sweden, who ruled from 1599 to 1611, and his first cousin was Gustav II Adolf of Sweden (1594–1632).