Kazuna is a young man with a fairly normal life. He attends high school, lives with his surrogate family, and models for his somber love-interest Yaegashi's paintings. But recently, he has suffered several crippling attacks at the sight of blood - attacks which leave him incapacitated and out of control. Kazuna must now reunite with his sister who he has not seen in years, and discover the truth behind his family name and vampiric genes, before his bloodthirsty desires destroy himself, or others close to him.
Hitsuji no Uta focuses more on the conflict caused by the "vampire" afflictions of the main characters, whereas Tsumi Ni Nureta Futari actually focuses more on the consequences caused by a taboo relationship. Both contain a forbidden relationship between an older sister an a younger bother, and are good pairs in the sis-con genre.