Tsuma no imōto: Ikenai yokujō (2003)

Yusuke is having an affair with a female colleague because his sex life with his wife, Jitsuko, has become stale. One day, upon returning home, he finds his wife's younger sister Mai in the bath. After becoming aroused by peeking at her bursting, naked body, Yusuke finds himself being aggressively pursued by Jitsuko...