There are other comments stating this, but the various typos and incorrect pronouns makes it hard to tell who's saying what a lot of the time. I assume its due to translation issues, but the overall message and story is still mostly understandable.
Overall the story had good Yandere elements, and all the onii-chan stuff made the 180 change into crazy that Manaka had compared to the start and the insinuated past history the Protagonist had with her more creepy and uncomfortable. Good work!