Shinde Hanami ga Sakihokoru

Sakurasuke Hanazaki lost his family in a fire when he was young and has been alone in the world ever since. He was taken in by the distant relatives of the Haruhara family and lives with Haruhara's parents and siblings, Yoichi and Yomari, as if they were his real family. One day, at the summer festival at a shrine where he went with Yoichi and Yomari, they are attacked by mysterious creatures created by a young man named Manshu who kills people...!?

(Source: Kodansha, translated)