Manbiki Musume Momoiro Seisai - Manbiki Shita Imouto ni Kichiku Oshioki!!

One day, the main character, Fujisawa Takuya, receives a phone call from the manager of the supermarket where he works part-time. He is told that his younger sister, Fujisawa Chisato, has shoplifted. When confronted, she begs her brother not to tell anyone. Takuya agrees but plans to use this to his advantage: that is, using his younger sister's now completely feminine body as an outlet for his sexual desires.

Chisato is suddenly attacked in the bathroom, an act that was nothing but humiliation for her. Time and time again she was raped: in the living room, at school, even outdoors. She slowly felt the bitter pleasure, and soon started drowning in it...

[Translated from DLsite]