User blog comment:Merganafan23/Character ages/timeline/@comment-5102537-20120804234140/@comment-5102537-20120805094454

It's a shame that they never clearified all those questions concerning Morgause/Morgana/Vivienne/Gorlois. Maybe it's indeed just a plothole. If it's not, then maybe Morgause was smuggeled out due to different reasons since Gorlois wasn't her father. She can only be Morgana's half-sister through her mother, otherwise Gaius, Morgause and Morgana would have thought that they were full sisters before Uther told Gaius that he was Morgana's father. Vivienne seem to have committed adultery not only once but twice. Though it could be that she was married to another man before she married Gorlois and Morgause was the child of her former husband. But if she had committed adultery, Morgause was an illegitimate child and therefore was smuggeled out of Camelot while Gorlois and everyone else, including Uther were led to believe that she had died. Maybe her inherited magical powers made her important to the Old Religion which was the reason why Gaius had to swear an oath so that no one tries to find her. We don't know what other people and kings thought of magic back then, maybe Vivienne and Morgause were seen as a threat to them and therefore in danger. Magic corrupted back then and almost destroyed the Five Kingdoms after all.

Either way, Morgause is definitely older than both Morgause and Arthur. I don't know if she told Arthur the truth about knowing Igraine. If she did she probably met her as a child. Or maybe she had access to the afterlife.