Kono Naka ni Hitori, Imouto ga Iru!  

My Little Sister is Among Them!, Who Is "IMOUTO"?, NakaImo, この中に1人、妹がいる！

According to your father's wishes, you were transferred to an institution where many females are enrolled. Your objective while attending this school was to find a romantic partner. On your birthday, someone sent you a cake from an unknown source. Then, you received a phone call with the message "Happy Birthday, Big Brother! I love you so much." It seems that your little sister, who was separated at birth and also attends this academy, is trying to get closer to you without revealing her identity. Can you successfully reunite with your sister and find a romantic relationship?