Kono Naka ni Hitori, Imouto ga Iru!

The story centers around Shogo Mikadono, the son of an industrial magnate who has just passed away. As his dying wish, Shogo's father gave him a condition before he could inherit the family fortune: To find a girlfriend at school. Shogo immediately returns to school in order to fulfill this condition. He then learns that he has a younger sister from whom he was separated at birth. The only clue he has: she is supposedly a student at his new school…