CloudCity

First and foremost, It's your Dad's girlfriend so it's HIS problem. The time for you warning this little bitch rule your life is over. First, get locks and put them on all the doors in your part of the house since a simple "stay out and leave my shit alone" isn't working. Next, I'd be mean and do shit like break her stuff, hide her stuff, and make life as miserable for her as you can without making it obvious you're behind it. When it comes to the girlfriend and her worthless kids, I would ignor them, make them feel as uncomfortable as possible when they're in your home - make them feel like they're a temorary, soon to be gone nuisance in your home and life. If your Dad gets on you, then tell him to HANDLE THE PROBLEM! It is his problem after all. Be strong brother and do what you have to do!

Peace,
CloudCity