Thanks for reading.
Here we have, in the fairy tale/baseline arc, Morgause setting off to, ultimately, become the mother of Arthur's bastard heir - but in the space arc the half-sister concerned is Morgan. This deviation from Le Morte d'Arthur (Malory incidentally couldn't decide whether Morgause was Arthur's half-sister or his aunt) isn't original with me. It's a salute to my second-most influential modern retelling of Le Morte d'Arthur, Marion Zimmer Bradley's The Mists of Avalon.
My most influential retelling of Le Morte d'Arthur is T. H. White's The Once and Future King which, according to White a retelling of Le Morte d'Arthur without invention of his own*, contrives to leave Morgan le Fey out almost entirely. Thus The Mists of Avalon, retelling Le Morte d'Arthur from the point of view of Morgan as the high priestess of the old religion while Christianity becomes entrenched in Britain, was pretty much my introduction to the character. Revisionist view of Le Morte d'Arthur that it is, it was also the work which brought home to me the way the legend lives on by adapting to successive cultures. The Mists of Avalon is the mother of Arthur, King of Time and Space in the same way that The Once and Future King is its father: Take away either of them, and there's no AKOTAS.
There's been a general trend in recent literature to replace or conflate Morgause with Morgan: John Boorman's film Excalibur, Mike W. Barr's graphic novel Camelot 3000, the Doctor Who serial Battlefield, The Road to Avalon by Joan Wolf (which I like just because Wolf keeps everyone but Agravaine from being a bad guy). You may know of more examples.
The difference in Arthur, King of Time and Space, in which sister is Mordred's mother when, is to make a profound difference on him in the space arc. For one thing, because of the space arc time anomaly, he's now old enough to be a squire on the Excalibur. In fact he's already appeared in the space arc more'n once. The parody arc, too. And all this is now covered in the FAQ.
* Except for the love affair for Pellinore, and "Malory did not say that
Lancelot was ugly".
Arthuriana sources I use or recommend:
Arthuriana - the Journal of Arthurian Studies; the website of the quarterly journal of the North American Branch of the International Arthurian Society.
The Camelot Project at the University of Rochester.
Camelot In Four Colors: A Survey of the Arthurian Legend in Comics
Mystical-WWW - The Arthurian A2Z knowledge Bank which has encyclopedically-arranged entries on the characters of the Arthurian legends.
Le Morte Darthur: Sir Thomas Malory's Book of King Arthur and of his Noble Knights of the Round Table, Volume 1 and Volume 2.