Bullied by my younger sister. Parents are rubbish - what do I do?

My younger sister is 18 (i am 20) and gets away with anything. literally. she broke into my dads flat in london and smashed the windows, and at first he said she couldnt ever stay in it again. a week later, shes staying there. after chucking a chair at me and threatening to kill me, she stole my keys to my dads and went and stayed there, and my parents did nothing. they just dont want trouble from her. if they refuse one of her demands e.g. letting her use the flat or giving her money, she threatens to trash their homes (they are divorced). and she actually would. i think shes mentally ill coz she literally doesnt have a conscience. my two elder sisters dont like her either but they dont really help me. they will stick up for me but they dont DO anything. currently my mum and elder sister are talking to her because she threatened to give my stuff to a tramp if i didnt return to the flat an hour and a quarter earlier than i was going to. but all they do is tell her they are there and feel sorry for her. there is no authority or discipline and she knows she can always do or get whatever she wants. the rest of our family is normal - ie. me and my sisters have a conscience and respect for our parents and would never disobey them to the degree of breaking the windows to their homes. she is just so nasty and horrible to me and tells lies about me to my family. she twists the truth and i just feel on the verge of tears the whole time around her. my best friend told me to ignore her, and im trying, but it doesnt stop the hurt i feel when my family literally DOES NOTHING to protect me from her bullying. there was no punishment after she threw a chair at me. they sometimes take money from her account but she has rich godparents and loads in her savings account so it doesnnt affect her. so basically i dont know what to do - clearly my parents and family are no help - so who else can i turn to? i wish the police would lock her up and put her in a mental ylum where she belongs. she once threatened to burn my dads house down. he is old and cant deal with the stress and im worried its going to make him have a heart attack, but i still half hate him for not sticking to his decisions and blaming me for her crap. please help me!!