Fuzoroi no Lemon

Machida Haruko, nicknamed "Faru", is a young girl who was living together with her sister Mei. Mei has mysteriously disappeared, and one day Faru received a package, which contained a video showing her sister, tortured by unknown people, crying in agony. The young heroine heads to the high school Mei studied in. She tries to get a job there, in order to investigate the circumstances of her sister's disappearance, and bring her home safe...

[From Moby Games]