User blog comment:Aithusa07/List of Plotholes from Merlin/@comment-123.2.187.141-20150530162132/@comment-80.194.173.91-20180131171338

You'd have to assume Uther was king by this point, Gorlois was a lord and a knight living in Uther's palace. Vivian, for lack of a better word, was an adulterer and a harlot, who must have had magic in her family. Gorlois might have been away or not even Vivian's husband by the time Vivian had a pre-wed fling with an unnamed lord. A scandal like this either way in Uther's court would have brought shame. So Uther and Gaius are in kahoots and once Morgause is born, Uther may have wanted Gaius to make Vivian 'lose' the baby, but instead the physician smuggles her out to be raised by high priestesses, due to Vivian's requests.

Next, some time later, Gorlois and Vivian are married and Gorlois goes away to war. Maybe they are together the night before he goes, and she 'grows lonely' a few days to a week later, for Uther to fall to temptation (to keep the timing of conception right). Uther may by this point be frustrated he's not getting a child out of Ygraine. Gorlois comes back, Vivian says she's pregnant and no-one else suspects he's not the father. Months go by and maybe the couple moved to their own castle (Uther says Morgana 'comes' to him after Gorlois' death later) and soon Uther asks Nimeuh for magic and Ygraine is pregnant. and then we know the story onwards.

People say the casting for the two half sisters could not make them look more different, but maybe Morgause got Blonde hair from the unnamed lord and brown eyes from Viviam, and Morgana got dark hair from Vivian and hazel eyes from Uther. But for Morgana to first lay eyes on Morgause and think she's met her before was a bit silly.