[7 Rensa (Takamichi)] Imouto Netori -Watashi ga Saki ni Suki Datta no ni- [Digital]

The little sister is like those guys who loves a girl, but can't express her feeling until seeing the girl being taken by other guy. Only in her case, rather than just crying her fate, she decide to seduce him, even if it means hurting him with guilt (of betraying his girlfriend for her sister).

Yuzuru, the MC, is dating Kiyoho after he helped her from being groped in train. He later found out Kyoka, his classmate, is Kiyoho's younger sister. After the kiss, both starts doing more behind the girlfriend. Actually, Kyoka had loved Yuzuru first when he tried talking to her in class. She got no friends. Later, she found out her crush is her sister's boyfriend (and the sister brags a lot about her prince charming), and Kyoka also got groped in train without anyone rescuing her. So she took the move. Kyoka threatened to tell her sister about them having sex, in exchange, she wants Yuzuru to keep the relationship with both sisters.