Katanegai

Upon learning of their parents' passing, the Minami siblings—Seia and Nagi—returned to their ancestral village.
With few villagers remaining, they settled into a quiet life together...
But when Nagi fell gravely ill, Seia sought help at the shrine—where he encountered a mysterious black-clad shrine maiden named Yami.
To save his sister, Yami revealed that he must "unite" with Nagi.
This is a tale of defiling wishes to fulfill the—all to save the one most precious to you.