AKA @queer-ragnelle is right: Every Vulgate writer listens to his own canon.

Here, Igraine is stated to have 2 husbands before Uther, and five daughters as a result.

It is unclear which daughter was from which husband.

Curiously, King Aguisant is presented here as another nephew of Arthur's. I wonder if the Caradoc stated to be his father is Caradoc Briefbras, King Carados of Dolorous Gard or some other Caradoc inspired by those two.

The fifth, and presumably the youngest, is implied to be Morgan, the convent girl.

Here is where Vulgate cycle will throw anyone who read Le Morte for a loop: Morgan is NOT Ywain's mother. It's an entirely different sister.

This, in turn, is actually in line with the implications from earlier stories, like Knight of the Lion: Morgan wasn't Ywain's mom until Post-Vulgate, this change being likely inspired by Welsh Folklore, where the mother of Owain mab Urien and his sister was Modron ferch Afallach, an Otherworldly Lady.

In other words, Morgan's only similarity to Modron was a late invention, inspired by Modron's own relationship to Urien. Urien himself was never Morgan's husband in the beginning.

[....]

Here is another example of the Medieval writers contradicting themselves: Blasine, wife of Neutres and Mother of Galeschin, being the daughter of the Duke of Tintagel, instead of Igraine's purported first husband. Galeschin compounds the contradiction by saying Uther is the second husband, negating an earlier husband before the Duke.

And finally, an interesting tidbit: Igraine herself is not the Duke of Tintagel's only wife. Igraine is his second wife. This son who inherits the dukedom is unknown, but my own speculation hints it might be Cador, but that has its own problems.