Haitoku

Koichi lost his mother as an infant, and his father remarried a woman with three daughters: Misaki, Haruka, and Ayano. Unfortunately, Koichi's stepmother is killed in a traffic accident, and because of his job, his father is constantly on the road. Koichi and his stepsisters help each other and build a relationship that is even stronger than if they had been real brother and sisters.

One summer day, Misaki suddenly disappears. Later, they receive a mysterious videotape. They play the tape and see Misaki being violated. The attached blackmail note says, "If you want to save Misaki, rape the other sisters." With nothing to lose, Koichi decides to train his sisters as sex slaves.
Now, their summer vacation turns into a sexual training camp. Who is threatening them? What does the person want? What will happen to this family?

Under the theme of 'immorality,' witness our hero raping his sisters. By raising parameters, such as 'affection,' 'sexual feelings,' and 'immorality,' the varieties of sexual training will increase dramatically.

[From Gamelex]