Osananajimi no Kanojo ga, Giri no Ani to Futari de Kurashi Hajimeta Ken ni Tsuite

Eri was a bright and popular girl with style and friends.
This story is about her.

Eri's neighbor was a boy two years older named Shogo.
They were good friends and over time became lovers.

One day Eri's parent got remarried, and she gained a new brother, Masao.
Masao was atrocious and cruel, making her his slave. Unlike sex with Shogo,
she felt no passion at all with him. But over time his force broke her...
and then she learned that she loved being mistreated.
In the end, her heart and body are surrendered fully to cruel Masao.

[From DLsite English]