The Once and Future King Book 2, Chapter 14
The Questing Beast finally believes Sir Palomides' explanation of who her "mate" really was (Palomides and Grummore dressed as a beast!) but then she falls in love with Palomides himself. She is no longer interested in Pellinore, even though he has chased her for eighteen years, so from then on Palomides "hunts" her. There is a fabulous double wedding at Arthur's castle, with Pellinore and Toirdealbhach and their new wives. Merlyn is far away, on a vacation, but he wakes up suddenly, remembering what he needed to tell Arthur: who his mother was! Arthur's mother, Igraine, was also the mother of Morgause, Morgan and Elaine-the three sisters whose father was killed by King Uther. Uther had fallen in love with Igraine, and forced her to marry him, even though she already had those three daughters. Arthur was the child of that marriage. No one knew where Arthur had been taken when he was born but Uther and Merlyn-not even Igraine. Now Merlyn remembers that he never told Arthur this, but he is sleepy, and figures he can tell him in the morning.
Now it is too late, however, because Morgause-Arthur's half-sister-has come for Arthur. She appears before him, beautiful, with her four children behind her to make her seem more appealing. Arthur falls for her, and she has his baby nine months later. She names him Mordred. Thus, Arthur's family tree includes incest, even though he did not know it at the time. "But it seems, in tragedy, that innocence is not enough." Book 2, Chapter 14, pg. 323