Kono Naka ni Hitori, Imouto ga Iru! (Novel)

Mikadono Shōgo, the son of an industrial magnate, has until the end of high school to find a girl he loves for marriage because of his late father's last will. For that, he transfers into Miryuuin Academy. Among the students there is a girl who says that she is Shōgo's separated-at-birth sister, but she does not inform him of her identity. She and some other girls then try to get close to him.