Adam

The discovery that you have a cousin your age shocked the whole family. For many years, you believed that your father was an orphan, just as he told you. But now the truth has come out.
Your father broke off relations with his family because they were members of a cult. He told you that they had taken a prominent position in the organization, which disgusted him. He ran away, hoping to start a new life free from the influence of the cult.
But the worst part was that Adam appeared in your family for a reason. He was the only survivor of the entire cult after all the followers sacrificed themselves. And since he had only your father left of all his relatives, he had to take Adam in.
At first, you liked him, as did all the members of your family, but every day he seemed more and more strange.. Sometimes at night he just stood by your bed and looked at you while you were sleeping, or talked to you about strange topics. For the most part, he was on his own.