My family is falling apart, what should I do?

My family use to have a good relationship, but ever since my mother had spinal surgery in 2005 my family has not been the same. My moms personality has changed the most, she has gained weight, smokes a package of cigarettes a day, she has also became more snappy and yells at me and my younger brother every time we do something wrong. My step dad has also changed he has become more depressed and he wants to leave my mom but he won't because he doesn't want to leave me and my brother without a home. My little brother has changed also, he has gained weight. But this is tearing me apart the most I've been cutting for like 3 months now and I've tried to stop but I can't. I am also suicidal, I sat down for 3 hours last night holding a knife to my throat thinking of life. I can't take it anymore! I've tried talking to my parents, but they won't listen to me. The only 2 things that have been keeping me alive is my little brother and my favorite band in the world My Chemical Romance. What should I do? I'm hanging by a thread, I don't know how much longer I can take it.�