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 has dark, psychological themes that involve a forbidden relationship between a brother and a sister, which are prevalent also in Flowers of Evil. Both manga also contain similar story elements as well, but Hitsuji no Uta has an additional "vampire" element.
Though she's never spoken to him, Wana Koizumi has a huge crush on a man that frequents the restaurant where she works – but little does the girl know that he's actually a vampire and is interested in her too, due to her resemblance to a woman he loved long ago. One fateful day, Wana is working at the restaurant as always when her parents decide to pick her up from work, for recently they have heard of disturbing murders that leave their victims drained of blood, and they are concerned for Wana's safety. However, on the way home they are involved in a fatal car accident that tragically kills them – that is, except for Wana, who wakes up on her crush's sofa covered in bandages and harboring a newfound thirst for blood...
The lead characters of Lament of the Lamb and Vampire Girl become vampires and it is the worst thing ever. They can't control their bodies, and feel completely helpless. Both manga also explore "replacements", creepy co-dependant relationships, and unearthing dark personal histories with a more somber mood and less action than most vampire stories.
Vampire Girl has a much more substantial plot, whereas Lament of the Lamb is pure atmosphere, and has a more interesting take on vampirism.
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.