Sisters a *****, advice?

My sister has broken up with her boyfriend and has moved back into the house (she's 18), I'm 15 and our mom who broke up with out dad when I was 3 is all over the place and needs help round the house because her job is extremely stressful and she works for 5 days of the week from 7am to 2-3am (it varies). Ever since she's moved back she's become more of a ******* lazy self centered egotistical cow than she ever was. She was diagnosed with OCD and ADHD a month ago (bullshit to me) and think it's an excuse to do anything. She's no vain and declares how beautiful she is and how ugly I am etc. We were beaten by our dad when we were younger (I received a punch to the face and the lash of a belt) and she got a slipper on the **** and claims to nearly every single person she meets how bad her childhood was and how I got the easy life and was favoured above all. I was dyslexic and had to be tutored for a year or got to remedial school. Anyway, my mom always bought her gifts and paid everything for her from Irish dancing, ballet, tennis, guitar, singing, netball etc. and bought her make up shampoo expensive clothes all the time. I got a few things a year and was grateful, I even taught myself the guitar. And she used to beat me up when I was younger and get her friends to join in as well. I wouldn't put shrek 2 on for her so she kicked me down the stairs and pulled a knife out on me and scratched my arms to piece. I pushed her over AND GOT THE BLAME. My mom even payed for three driving tests, permits licenses etc. and she failed every single test. She's even gone out tonight at 12:00am to cheat on her boyfriends and when I asked why she said "Because I'm a knob and started laughing. She would always lie about how I used to beat her and say things like "You;re the reason mom and dad got divorced, you;re the second unloved child etc. You're ugly, no one loves you" and I'd always feel like **** after wards. My parents divorced on my birthday so it was pretty rough. And she tells everyone her life story and how She feels all the time and it's pathetic, I don't tell anyone noe even my best friend. Anyway do you have any advice on how I can cope?