Hitsuji no Uta

Kazuna's family carries a curse-a disease that causes an insatiable, vampiric hunger for blood. Unaware of this dark legacy, Kazuna was raised by the Eda family. But when symptoms of the illness start appearing in him, he crosses paths with his older sister, Chizuna, who has lived with the affliction since she was young. Knowing the torment it brings, Chizuna tries to lighten her brother's burden and teach him to resist his urges-but the struggle is far from simple.