Oh No! my cousin is a Zombie!

You are a highschool guy living with your parents. One day your family had to travel to work abroad so they sent you to live with your aunt who also has a daughter that is younger than you. Your aunt had to work for a week outside town so she cannot take care of her daughter, so she asked you to do that instead. One day you realize your cousin is adopting a strange behavior, as if she was not herself. You find out later, that she turns into a zombie at night so you tell your aunt but as expected...