Inshitsu Otaku ni Ikareru Kanojo

Shuji lives a peaceful life with his younger sister, Saki, but he harbors a secret love for her that grows increasingly uncontrollable. One day, Saki confesses her own long-held feelings, and they begin a romantic relationship, finding happiness together.

Their joy is shattered when Shuji receives an anonymous email containing a photo of Saki changing clothes and masturbating. The sender, a stalker, threatens to expose their secret and begins issuing Saki a series of immoral orders. Though she initially resists, Saki gradually becomes addicted to the stalker's demands, and Shuji must find a way to save her before their world falls apart.