How do I tell my mum that I can't cope any more?

both my mum and my step dad have multiple sclerosis, and they are always tiered and narky, fair enough, i understand that they have a reason to be. but it seems as though anything i do isnt right. my step dad can barely walk and is a dead weight to our family, and to top it off, he is from a different culture and our personalities crash really bad. my parents broke up 6 and a half years ago, it was a really bad break up and my mum has complete custody of me. i am currently starting the most important year of my life, year 12, i am in VCE (Victoria, Australia, schooling system). i currently work up to 15 hours a week, and i am severely anaemic myself. i cant live with my dad permanently because he lives about an hour away and the distance between them is to difficult to travel too and from school everyday during this final year of school. i ran away earlier this week and im avoiding talking about it with my mother because she will just yell at me and we will en up being even angrier with each other. i want to go to my dads house every other weekend, i want to be able to do homework, but i cant because i have to vaccum or clean my room. the attitude that i am getting from my mum at the moment is that bad that i have thought about commiting suicide, and even one day i actually wrote the note and set it all up. the only thing that stopped me was that i got a phone call from my dad, it was my little sister, she just turned two, and she told me she loved me. at the moment i only get to see her on holidays and birthdays, she is growing up so fast and im missing out on it. i know that suicide isnt the way out, and i have come to the conclusion that it isnt right to be having those thoughts, and i am trying hard everyday, and i would be getting past it, but the way that i am getting treated from my mother, step-father and sister is that shocking that i cant get past what is happening to me. i just, the main thing that would stop this is … well, i dont know. but i dont want my life to be ike this, i dont want to live the life im living, but im a minor. i have no say. any advice would be greatly appreciated.