Morgan le Fay
According to Geoffrey, Uther and Igraine were parents of King Arthur and a daughter named Anna, who married King Lot of Orkney. Chretien de Troyes' introduces Morgan le Fay as the sister of King Arthur. In Gerald of Wales Tour of Wales (1188), he has Morgan as Arthur's cousin. Malory credits King Arthur with three half-sisters: Morgawse, Elaine (Blasine) and Morgan le Fay. Answer by Plucas. Confidence votes 31.4K. Later legends say that Arthur had three half-sisters: Morgawse, Elaine (Blasine) and Morgan le Fay. Morgawse had married King Lot of Orkney, Elaine (Blasine) was married to King Nentres of Garlot, while Morgan was wife of King Urien of Gorre, brother of Lot. Later legends say that Arthur had three half-sisters: Morgawse, Elaine (Blasine) and Morgan le Fay. Morgawse had married King Lot of Orkney, Elaine (Blasine) was married … to King Nentres of Garlot, while Morgan was wife of King Urien of Gorre, brother of Lot.   But Morgan Le Fay was the best known. Morgause does not appear in the historical texts. Geoffrey of Monmouth says Lot was married to Anna, Arthur's half-sister, not Morgause. But Malory introduces the Morgause as being wife of Lot, King of Orkney. In the Welsh romance Gereint and Enid, the story mentioned that King Arthur had a son named Amhar. His most noteworthy son was Mordred. In the early chronicles like Geoffrey of Monmouth, Mordred was Arthur's nephew, because Mordred was the son of King Lot and Anna or Morgawse, the sister of Arthur. But there is frequent confusion of who were Arthur's sisters and who was mother of Gawain in the mainstream Arthurian legend. According to Geoffrey, Wace and Layamon, Uther and Igraine were parents of Arthur and a daughter named Anna, who married King Lot of Orkney.
http://www.legendofkingarthur.co.uk/genealogy.htm http://www.answers.com/Q/Who_was_king_arthur%27s_half-sister http://www.answers.com/Q/What_was_king_arthur%27s_sisters_name http://www.legendofkingarthur.co.uk/legendary-characters/morgause.htm http://www.legendofkingarthur.co.uk/genealogy.htm http://www.timelessmyths.com/arthurian/arthur.html
who was king arthur's sister