Mama to Imouto to Boku to Sensei

The protagonist's mother is an erotic novelist and she is commited to write a new story with the help with a teacher. But in order to have new material and better sexual ideas, she is violated by the teacher. And now, the teacher violates his younger sister too! Teacher is using his mother and his sister as sex tools. Who will you defend from the hands of this terrible demon? Your mother, your younger sister or both? Are you really fit to defend them? [Translated from Getchu.com]