Diseased

y/n is the Sakamaki's greatest secret. She is their little sister who they would do anything for, and they have. Somehow they have managed to keep her a secret from Karl Heinz for thirteen blissful years. But the happiness ends when, on her thirteenth birthday, she is suddenly struck ill with a deadly disease. They are able to slow it down but they can't stop it, two years pass, and just when it seems they will lose their beloved y/n forever, a girl comes to the mansion and somehow y/n is improving by the day.
How is this girl the key? What caused y/n to get so sick? Will the brothers be able to stop themselves from killing the girl, in order to save their sister's life?

There is only one way to find out.
(A/N) This is my first fanfiction so I'm sorry if it sucks please give it a chance, I do not own Diabolik Lovers, just any characters I create and add.