Chocotto Sister (2006)

Original title: ちょこッとSister

Haruma Kawagoe is an only child. A long time ago, around Christmas, his mother miscarried the child that would 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 about his prayer. But Santa did not... so one Christmas, when Haruma is least expecting it, he receives an unusual present — a sister named Choko.