Diabolik Lovers: The Hidden Vampire Princess
The Sakamaki brothers have a younger sister who's precious to them and she's in a deep sleep, her name is Sakamaki Airi, a 15 years old girl and the daughter of Karlheinz and the deceased Elaine (the 4th wife of Karlheinz).
The only one who could wake her is Karlheinz but there's a prize that must be paid. The Sakamaki brothers must obey and follow Karlheinz's Adam and Eve project.
The plot's originally mine, I've found someone who copy some chapters of mine so they're really similar and that story is "The Sakamaki Princess", I've asked the author to not copy mine a hundred times and rewrite the plot herself but she wouldn't listen and kept ignoring my messages, I am actually quite disappointed. Even until now, I really wished the author will change and make her own plot.