FATE/KALEID LINER PRISMA☆ILLYA: LICHT NAMELESS GIRL

The world is dying, no longer able to support life. The Ainsworth family can use the Holy Grail to save humanity, but the price of salvation is sacrifice of the magical girl Miyu — but Miyu’s brother, Shirou, won’t let them have her without a fight. Now it’s up to Illya to step in and find a solution that will save not only Miyu, but the entire world for impending doom. But when the price for the world's salvation is the life of a single, innocent girl, what will Illya choose? (Source: Sentai Filmworks) — Description from AniList community (anilist.co).