Looking for a Manga

A girl found herself possessing the main character’s step sister in a story and the main character in the story was a psychopath who would do crazy things to keep himself entertained because he was bored with life (I believe that was the reason). The girl tries to appease her step brother by acting spontaneous because she knows if she allows him to become bored with her the original story will play out and end up with her getting killed by him.