I cannot stand my boyfriend's sister, what do I do?

My boyfriend's sister is 16, I'm 17, and my boyfriend is 20. She and I used to be best friends but it turns out she was only doing it to try and break her brother and I up, she failed and we quit hanging out. Now she does everything in her power to make my life miserable, but its not just me she does it to everyone. She gets everything handed to her and she always wants and needs something and when she gets it something isn't good enough. She is never satisfied, she is wicked two faced and she's a total backstabber. She gets into my boyfriends business, especially when it comes to money. She's bother by our life and she expects us to never have when we are visiting, even though we spend every second we can at his mother's house because my boyfriend feels he doesn't see his mom enough. His sister does whatever she wants whenever she wants and she refuses to listen to anyone. She claims she's more mature even though she throws a fit when she doesn't get her way. She likes to think she's got the maturity level on an adult although she really has the maturity level of a 2 year old. For example today when she had to sit in the back seat of the car she kept punching the doors and kicked my seat. My boyfriend is always trying to make excuses for her and no matter how many times I tell him there is no excuse that she's just an immature, whiney, spoiled little ***** he just tries to tell me its something else. I can't stand her attitude anymore, I just wish I could punch her in the face everytime I see her. And shes always got the same look on her face the "woe is me, my life sucks, I never get what I want, everyone's so mean to me" look. I'm 2 seconds away from beating the **** out of her, help?