Hitsuji No UtaLament of the lamb; 羊のうた

This is about two siblings with the vampire-like tendency for blood, and at first share the same home alone till their caring aunt decides to move in with her husband to care for them, however their normal life is going to get far crazier as a doctor and other characters become involved in the plot. From Tokyopop: The members of the Takashiro family share a terrible curse: only human blood can slake their thirst. Sent away after the death of his mother, Kuzuna Takashiro remained blissfully unaware of his 'condition.' When Kazuna's teenage hormones begin to rage, his uncontrollable bloodlust suddenly rears its head...