My 27 year old sister is an absolute Nutter, what should I do to knock some sense into her?

My sister who is 27 acts like a 12 year old. She works a job that pays $9 an hour, and relies entirely on my parents for everything. She has no bills, and has nothing tying her down. She watches 7 hours of television a day, and while watching television, she is playing farmville, fishville, mafia wars, etc. She has laid claim to an entire section of my parents house, and yells at people when they try to use it. She honestly believes the area surrounding the only good television is her domain, and has done everything possible to lay claim to it. She put 2000-3000 dvd's next to it, and has her stuff thrown across the only couch in the room. Her room is a disaster, and looks like a dumpster threw up on it. She has let her room fall apart, and has more clothing than half of California. Since she has no expenses she spends all of her money on getting drunk and buying crap that she does not need. My parents allow it to happen, because they have given up on trying to convince her, and they are too cowardly to cut her off financially. I am 20, and on college, thus I can only stay away from the house for so long. My sister drives me insane when I'm around, and makes me want to kill her. On a few occasions, sitting in the room that she has declared hers has led her to try to kill me, by throwing metal objects at me. She is completely nuts. What should I do? My parents won't do anything, and I am losing my mind. Would I be a terrible person for telling my extended family what is going on, to try to get them to knock some sense into her? I'm sure they have figured out something is not right since she isn't working a real job, and has made no efforts at getting one. What should I do?