PUPA

The "life-and-death sibling" story follows Utsutsu and Yume Hasegawa, a boy and his little sister who find themselves all alone. One day, Yume sees a mysterious red butterfly and her body undergoes a strange metamorphosis—into a creature that eats humans. Utsutsu struggles to find a way to restore his sister. (Source: Anime News Network) — Description from AniList community (anilist.co).