Shukujo no uragao: Abakareta chishin

One morning after a night of drinking, Tomoharu spots a man and woman entangled in an abandoned building. He is excited not only by their lewd behavior, but also by the man's voice calling the woman's name, "Eri." Six months later, a friend asks him to check on the safety of his missing sister, Natsuki. The photo he is given looks exactly like "Eri."