DEATH GAME MANGA NO KUROMAKU SATSUJINKI NO IMOUTO NI TENSEI SHITE SHIPPAI SHITA

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.(Source: Kadokawa, translated) — Description from AniList community (anilist.co).