Kono Naka ni Hitori, Imouto ga Iru!

Abiding by his father's will, Shougo was transferred into an academy where many young girls attend. It appears as though his younger sister (separated from him at birth) also attends this academy, and that she is trying to get closer to him without letting know him who she actually is. Problem is, Shougo seems to be popular with the other girls as...