Why did Morgause sleep with Arthur?
Similarly, you may ask, why did King Arthur sleep with his sister?
King Arthur Sleeps with His Sister Uther fights a war against Gorlois (in which Gorlois dies) and Uther tricks Igraine into sleeping with him. She conceives a son, Arthur. So they were agreed, and he begat upon her Mordred, and she was his sister, on the mother side, Igraine.
Also to know, how is Morgause related to Arthur?
Morgause was the half-sister of Arthur who married Lot. She was the mother of Sir Gawain, Gaheris, Agravain, Gareth, and Mordred. According to the Enfaces Gawain, Lot was her page with whom she had an intrigue, as a result of which Gawain was born.
Morgana, also called Morgaine or Morgan, is a staple figure of the Arthurian legend. Her relationship to Arthur varies but usually she is introduced as Arthur's half-sister, the daughter of his mother Igraine and her first husband Gorlois, the Duke of Cornwall.