Vampire Stories Brothers

What would you do if you found out that you were turning into a vampire at the age of 20? Would suck an innocent human's blood to complete the transformation or would you die not wanting to drink innocent blood? Vampire Stories Brother tells a story about Sei who finds out that he is a pure blood vampire from his older brother, Ai, and that he will turn at the age of 20. Sei has to choose to live or die and what lies in middle of his decision is his younger step sister, Midori, whom he cares for. What will Sei choose? Midori, his younger step-sister or Ai, his older vampire brother? To live or to die?