Long time for a stranger, short time for a bomb

How can a person love their life, and yet hate it so much? I love school, for the first time in a long time. It gets hard to wake myself up and follow the same routine, but I manage. Cosmetology fits me. It's more then just hair and make-up. I have a chance to make people feel a little better about themselves. That part of my life is great. It all makes sense to me. I get how that works and how it fits into my life and future plans. I can't say anything similar for the rest of my life. Nothing fits together. My sister gave her kid, a 15year old trouble maker in girls jeans, to my parents and it's all going down hill. I can't stand him. I hate having him live with us. I can honestly say that he's a bad kid! Smoking, drinking, drugs, sex, lies, cutting class, failing grades. Where does it end? When I was in high school I was not perfect. As an adult I can admit that, but how can you let a kid who's been caught doing all of these things get away with it?!? It's almost as frustrating as the religion Oprah started (look it up, shes kinda crazy!!!). I feel a little silly thinking I would buy this house from my parents and live here. Now I just want to move out and run away. I was so close to my parents, then they moved him in. They always pick his side and even when he steals from me they just think I'm paranoid and need therapy. I feel like I sound so childish. I wish I could change the way I feel about all of this but I can't. I've changed the past few months. I'm moody, and depressed, and feel alienated in my own home. Some 15 year old thinks he can boss me around and even though I'm his aunt treat me with total disrespect and threaten to slap me in the face. Of course I stand up for myself, but then my mother tells me I'm being a brat. I don't think I can stand this much longer. I am so worried that I will have to pay to take a leave from school to get a job and my own place. Too bad that I don't have a car because they sank money for a new one of trying to fix the old one. In case you were wondering, the car got worse and I don't have one now. I just feel trapped. Even screaming doesn't release the pressure in my head anymore. As an adult I don't know why I allow myself to be put in this position. I feel like I've turned into a powder keg, and at any moment I could run out of fuse. What happened to me? God, I need you're help!