Sweet rooM ～やって!やられて!?お姉ちゃん～

One day, the protagonist accidentally peeps on the place where Mayuri is bathing.
However, Shiori sees this and can tell about it.

"If you want me to keep it secret from your sister, you must become my slave". From that day on the main character endures Shiori's sexual torment.

[From DLsite English]