Cantarella - Ao no Dokuyaku

Legend tells of frightful Borgia family, and the poison "Cantarella" that is passed down from successor to successor...

The eldest son of the Borgia family, Cesare, is to be the next successor. Rejected and ostracized by the public, Cesare's life is a lonely one. But his unexpected salvation may possibly come in the form of his younger sister, Lucrezia, who had been separated from him since birth.

Indeed, Lucrezia's return to the castle could have been a new beginning for him, if not for his terrible secret...