Onee-chan no Kawari ni Nuite Agemasu ~Kanojo no Imouto wa, Shikatanai Kara to Zettai ni Yarasete Kureru~

The protagonist, Uchiki Kyouya, is frustrated that he still hasn't lost his virginity to his girlfriend, Mikuna. He turns to her younger sister, Moeru, who is his masturbation buddy. They made a promise to each other to masturbate together after Kyouya witnessed her pleasuring herself while sniffing his jacket. Although Moeru resists at first, Kyouya believes she will eventually give in to his advances.