Arthur's Little Sister

An alternate universe, in which Arthur has a younger sister, Elsa. She has magic, but no one can know. Morgana helps Elsa in some situations, some, not so much, and the same goes for Arthur. But this is the story of them getting through life, as children, and as they grow older. But one day, something happens, in Elsa's later life, that causes a drastic change (and the whole plot).