Aneki no Kounai Kaikinbi: 1x1

Aneki no Kounai Kaikinbi - Episode 1

Hirose Anna, 23, discovers that her younger brother masturbates to her filthy underwear and decides to stay one step ahead of him by allowing him to see her nasty side and escalating her activities each time. She eventually comes out and tells him that as his older sister, she will look after his sexual curiosity: she will let him do anything except vaginal intercourse, and while his requests become dirtier with each passing day, she accepts responsibility as his older sister, stands by her word, and keeps going.