User blog comment:Merlaa/Dear Brother/@comment-4650675-20121029131625

Morgana is so ironic and sarcastic when she calls him "brother". She does not believe Arthur is a real Pendragon worthy to be king, and much more someone who could be called her "brother". Arthur on the other hand NEVER calls her sister either.

See here why I think it is so. I think it's a good theory!