Guinevere is childless in most stories, In the latter text, Guinevere willingly becomes Mordred's consort and bears him two sons, though this is implied rather than stated in the text.
There were mentions of Arthur's sons in the Welsh Triads, though their exact parentage is not clear. A half-sister and a brother play the antagonistic roles in the Lancelot–Grail and the German romance Diu Crône respectively, but neither character is mentioned elsewhere.
Gwenhwyfach (also spelled Gwenhwyach) appears in Welsh literature as a sister of Gwenhwyfar, but Welsh scholars Melville Richards and Rachel Bromwich both dismiss this etymology (with Richards suggesting that Gwenhwyfach was a back-formation derived from an incorrect interpretation of Gwenwhy-far as Gwenhwy-fawr).
This was likely due to Chrétien's audience at the time, the court of Marie of France, Countess of Champagne, which was composed of courtly ladies who played highly social roles.
Most people, after reading enough fiction, begin to have an idea of how relationships begin to fit together, and can spot a budding romance a mile off.
Sometimes, though, the writers break away from these conceptions and do something entirely unexpected.
Regardless of how it happened, they managed to pull off a Relationship Writing Fumble and now the writers are stuck dealing with the consequences.
In minor cases, it will just be popular Fanon, but sometimes you'll have entire fanbases assuming that's what the writer "really" intended.