Inshitsu Otaku ni Ikareru Kanojo (Series)

Shuji lives a peaceful life with his younger sister, Saki. But he secretly loves her and his libido gets uncontrollable day by day. One day, Saki tells him, "I love you.... I've been always thinking about you since I was a kid...." Like this, they become boyfriend/girlfriend. However, their happiness is destroyed by an email. "I know your everything...." The email attaches a photo of Saki changing clothes and masturbating. Saki thinks her stalker did it. The stalker gives her various immoral orders. She first tries to resist it, but she gradually gets addicted to it.... Well, will Shuji be able to save Saki...? (Source: ErogeShop)