I Reincarnated as the Little Sister of a Death Game Manga’s Murder Mastermind and Failed

A brief description of the light novel I Reincarnated as the Little Sister of a Death Game Manga’s Murder Mastermind and Failed:
One day, Mai remembers that her perfect stepbrother is a character in a death game manga and that one year later he will hold a death game involving all of his classmates, killing his sister in the process. Mai plans a variety of surprises for her stepbrother every day based on what he said in the manga, “Nothing unexpected happened, so I was bored,” but as the days go by, he begins to show abnormal signs…
Only I know what my brother’s really like!