Kono Naka ni Hitori, Imouto ga Iru!

Shogo Mikadono - son of the powerful industrial tycoon Kumagoro Mikadono, who died prematurely. However, before his death Kumagoro time to pronounce his last wish - before Shogo continue the legend of his family, he must go to school and find a "helper" female. Following his father's will, Shogo transferred to a new school, but at the same time he learns that he still has a younger sister, with whom they were separated at birth. of course the guy has no idea how it looks, but it just so happens that it is one of the students at his new school.