What should I do

I need help, I know this but every one I speak to says it’s my own fault. I know it is really I do. I’ve got two and a half room mates I’ll explain in a second. One of them is great he’s a baker pays rent on time brings home bread for us to share, tells me if he’s going to have guests over for dinner, helps clean and maintain the house. The other is my older adoptive sister and her 2 year old daughter hence the half. She on the other hand left her husband before I went house hunting and joined along. She used to rent with me a couple years ago so I thought it would be fine. It’s been 2 months since we all moved in and because she didn’t have a job or Centrelink set up yet she told me that the applications had been approved and she just had to wait two weeks so I agreed to cover her rent and groceries for her and her child. She agreed to pay me back for it and to give me what money she could to cover… that was a month and a half ago and apparently she put in the forms the day we moved in. I can’t afford to keep three people fed and under a roof, I’m 20 now and I’m working 40 hours a week, my degrees gone down the drain and I’m in debt up to my eyeballs. She doesn’t help around the house and gets angry when I don’t make her dinner of a night because she wasn’t home. She leaves for hours at a time of a night no clue where she is and if I make her dinner and she’s had it she goes off for wasting food and If i don’t make her dinner she goes off for not including her and making her spend money on take out. I can’t win. I started this year in the best of ways and I was happy. I keep giving myself goals throughout the year to stop myself from doing anything stupid. Step 1 get a job, I got that done ok new goal. Step two go to uni, I got in and I was doing well. Step three get a motorbike so I can have transport. Ok done I now have my licence and finance on a bike cool. Ok step 4, find somewhere to live can’t keep sleeping on my friends couch. Done ok maybe a more long term goal because if things go wrong again I can fix it. Ok finish degree is a must, and go to Europe with a friend when I’m done. There that sounds good. I still have my job but I keep getting abused by customers the closer to Christmas it gets, I was punched a few weeks ago because I wouldn’t give someone a refund because he didn’t have a receipt or packaging. Like I said before my degrees going down the drain because I have to work so much. My motorbike got egged and then stolen two days later. So now I have to pay off finance on a bike I don’t own. Insurance said it’ll take six to eight weeks to even think about helping out. My sister spends all day watching law and order and criminal minds in the lounge room whilst barely keeping an eye on her child. I found her child on top of a book shelf watching tv in her play room when I got home last night, I had a heart attack and my sister said that she was fine with out barely taking a glace to the door way. My sister hasn’t tried to find work at all since moving in and apparently she’s waiting on Centrelink to approve her claim.. I just don’t know what to do. I feel so empty all the time, I feel like crying myself to sleep, I feel as if people only want me around when it’s beneficial to them. I feel as if I shouldn’t have been such a coward when I was 16 and just ended this. Every year, every god damn year I constantly try and every year I wind up back at square one. I just don’t know what to do.