User blog comment:Morganaforever/Are Arthur and Morgana siblings after all?/@comment-5674726-20121101214529

I can't imagine that the show would ever go there but it'd be amazing if they utilised this theory. It's a fascinating idea. Arthur is very like his mother physically, while Uther has commented on Morgana's similarity to "her father", as have others.

Perhaps it's also an explanation for why the price for Arthur's birth is a life, since you'd think that, if it was a case of Nimueh healing whatever problem kept Ygraine from conceiving, they wouldn't have to pay that price... unless, of course, somebody has to die for every child conceived after a parent's life is saved by magical means or they use magic to aid in fertility.