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

Here you can find a smiliar blog posts that might explain a few things.

http://merlin.wikia.com/wiki/User_blog:Emily_Windsnap/How_old_are_the_Merlin_characters%3F

Morgause is older than Morgana and Morgana is older than Arthur, therefore she was conceived before the Great Purge and there is no reason why Uther hadn't known about Vivienne's magical powers since he was friends with Nimueh back then and didn't have a problem with magic. That means that he actually must have known about Morgana's powers too, not to mention that her half-sister Morgause was magical too.

Moreover, the Great Purge can't be the reason why Morgause was smuggled out of Camelot shortly after her birth since she was born several years before Arthur.

If the show follows the common standarts of inheriting the throne in those times and even today, Morgana has no right whatsoever to the throne as long as Arthur or Gwen are alive. Therefore the throne can't be rightfully hers as she always claims.