She's currently 16.
She won't listen to a word my parents have to say. When they ask for the phone, since she's glued to it 24/7 and her grades have plummeted, she won't listen. She knows how to get on their nerves and knows how to make them angry.
Recently she's become even worse, despicable I might say. She's been blackmailing my parents. Every single time my parents ask her to do a chore, she won't listen. When they finally reach the point of frustration and start to get angry, she threatens them by saying "if you don't stop trying to make me do that chore, then I'm going to call the police". She has gotten away with it so far since my parents have had no criminal record and don't want to call the police for something that seems so trivial.
But my sister has been getting more and more aggresive, now using this to her advantage to get money for movies, tickets, and outings with her boyfriends.
What can we do?
Basically, she won't do anything they ask her to do. When they get to a point where they become so frustrated that she won't do anything, she'll hang the threat of "calling 911" over their heads as an excuse to get away with whatever she wants. Since I'm the eldest of the 3, I really have no idea what to do.