My Big Sister Lives in a Fantasy World: The Melancholy of the High School Girl Light Novel Author

Yuichi's not-very-restful summer vacation came to an end, and the new school term isn't looking much better. Immortal demons are trying to kill him, his new teacher is a superpowered sociopath, and all kinds of bizarre events are happening around the school. What does Kanako's new novel have to do with the bizarre goings on at Seishin High? Once again, it's up to the world's strongest little brother, and his highly delusional sister, to find out!