Question about King Arthur and Morgan le Fay

The family relationship became clear in the course of the 13th century in the Vulgate Cycle (in French). Most medieval texts after that keep it that way. In this tradition, Arthur has two half-sisters, Morgan and Morgause, and he sleeps with Morgause (NOT Morgan) not knowing she is his sister, and that is how Mordred is conceived. He also has a third, usually unnamed half-sister (Malory will name her Elaine two centuries later).

Prior to that, Arthur's sisters or half-sisters are only mentioned here or there and they play so little a part in the tale (when they do play a part at all) that it is difficult to draw conclusions with any definite certainty as to how they are related to Arthur exactly. Geoffrey of Monmouth mentions a sister called Anna, who may or may not be the same as Morgan. The only thing that is certain is that before the Vulgate Cycle, Mordred was just a nephew, not an incestuous son (or not even that: if you go back to the Welsh material, there is no indication of kinship at all between Mordred and Arthur).

"Occasionally I'm callous and strange."

WILDCHRISSY32

In the Mists of Avalon while it is more focused on the powerful women in Arthur's life they are in fact half-brother and half-sister until they are separated by Merlin and Vivianne (aka The Lady of the Lake ) so that Morgan can learn about the magic of Avalon and Arthur can be taught by Merlin how to become the right kind of King. In this instance Igraine is Vivianne's sister and to make things more interesting Lancelot is Vivianne's son. There is also Morgana's aunt Morgause who seems to have a darker plan. There is also of course the moment of incest brought on by magic in which Arthur and Morgana have sex during the Beltane festival which gives way for Mordrid's birth and thus King Arthur has a son by his half-sister and that son ends up being his downfall which is also in other Arthurian literature the Mordrid stuff anyways. But Mists of Avalon is great it's a lot of fun and a different take on things.