Ne midare hitozuma no imōto (2010)

Shinya and Sakiko married after a passionate romance that began in high school. However, after six years, a rift had formed between them. One day, Sakiko's younger sister, Minami, came to Tokyo. Shinya opened the bathroom door, thinking it was Sakiko, but he found Minami naked inside. The shocking image was burned into Shinya's mind...