Appetite - Anedori ~Ochiteiku Onee-chan o Tasukerarenai Boku~

The protagonist, Shirakawa Keita, lives alone with his older sister Orie, who just started her training as a teacher in Keita's school. They get along so well that his schoolmates believe they are married and thus shares the same surname, much to the jealousy of the male students.
One day, he received an unlabeled DVD from an unknown sender. As he played the said DVD, he saw his dear sister servicing someone's dick. At first, he thought it was her boyfriend, but soon he realized that she was being threatened to do it. The video showed her getting violated by different men, before it stopped.

He thought about talking to his sister about it, but he didn't have the courage. Instead, he decided to try and find the culprits alone by looking for clues in the video and searching around the school, but he failed. He then continued to receive similar DVDs of his sister on a regular basis...