They're Close

Prince Charles Darwin Phoebe is the sister of the twins, they have different mothers. If the twin's mother was a Devil, then Charles's mother was a Vampire. In the first place, Charles was not the one who would be the King of his Kingdom, it was all because it was the twins who would be his father's successor.

The reason for that was because that man was just a normal Vampire, in contrast to the twins who had Demon blood in their bodies. And his father got a prophecy that one of the twins would have to lead the Vampire Kingdom.

It was because a prophecy about a Lycan reincarnation would appear and that was why the Vampire King had hidden the twins in the human world. And because of that Charles also got a different treatment.

He was just a useless Prince because the King only thought about the twins. At first, Charles thought it was a natural thing because he was just an ordinary Vampire, moreover, his mother had also died after giving birth to him.