Kono Naka ni Hitori Imouto ga Iru!
Plot Summary: Shōgo Mikadono is the son of an industrial magnate named Kumagorō Mikadono who passed away. As a final wish, Kumagorō imposed a condition Shōgo must meet before he can inherit the family legacy—find a female mate at school. Shōgo transfers into a school to fulfill that condition. However, he learns that he has a younger sister from whom he was separated at birth. He does not know what his long-lost sister looks like, but she happens to be one of the students at his new school.