What relation was Morgan le Fay to King Arthur?

Morgan le Fay is a powerful enchantress in the Arthurian legend. Early works featuring Morgan do not elaborate her character beyond her role as a fay or sorceress. She became both more prominent and morally ambivalent in later texts, in particular in cyclical prose works such as the Lancelot-Grail and the Post-Vulgate Cycle, in which she turns into a dangerous enemy of King Arthur and antagonist of some tales.

The earliest accounts of Geoffrey of Monmouth in Vita Merlini and Gerald of Wales refer to Morgan in conjunction with the Isle of Apples (Avalon) to which the fatally wounded Arthur was carried off after the Battle of Camlann. To the former, in early chivalric romances by Chrétien de Troyes, she also figures as a healer. Her character may be partially derived from that of the Welsh goddess Modron and other myths. She is often said to be the daughter of Arthur's mother Lady Igraine and her first husband Gorlois, so that Arthur, the son of Igraine and Uther Pendragon, is her half-brother. She becomes an apprentice of Merlin and a vindictive adversary of Arthur and the knights of the Round Table, with a special hatred for his wife Queen Guinevere. In Thomas Malory's Le Morte d'Arthur and elsewhere, she is unhappily married to King Urien, with whom she has the son Ywain, and her sisters include Morgause. She is also wanton and sexually aggressive, with many lovers including Merlin and Accolon, and an unrequited love for Lancelot.