You can't be, Arthur's Uther's only heir. And Morgana... [Merlin trails off. There's really no way to explain Morgana - and Merlin's still hasn't quite fully processed the fact that Morgana is Merlin's sister either.
And that mention of a sister through her mother makes no sense at all] Who is your mother?
/gets out a broom to clean up all of these shattered brains
And that mention of a sister through her mother makes no sense at all] Who is your mother?