Chocotto Sister

Haruma Kawagoe is an only child. A long time ago, at Christmastime, his mother miscarried the child that was to have been his baby sister. That night, young Haruma knelt down and offered up an earnest prayer: "Please make my mother well again, and please give me a little sister." Years have passed, and Haruma has nearly forgotten his prayer. But Santa hasn't.... one Christmas, when Haruma is least expecting it, he gets an unusual present - his sister.
(Source:Anime News Network)