This is really urgent, i just don't know what to do please help!?

In my family there is me, my brother and the opposite sex twins. I'm the youngest and the twins are the oldest so my brother is in the middle. All of our lives me and the twins have been completely different to Peter, my brother, and recently as we've hit our teens, he's become out of control, but our parents don't so anything because they think he's just special. The three of us are over-achievers and he's an under-achiever because he has ADHD and dyslexia. He's grown out of control in the way that he goes out every night, he's in sixth form, and never studies or does his homework. He stays out as late as he can get away with and the latest has been 12 and this was on a school night. His highest GCSE grade was a B and his lowest was a G. He had sex with his girlfriend pretty early on because my sister walked in on them and who knows what else he's been doing because he's had a lot of girlfriends. He goes to party's and gets drunk, he's not even 18 and has collapsed because he has drunk so much. Recently my brother found cigarettes and a lighter in his room and my brother said that he smokes. He's ruined his body at 17. He always compares himself to me and my siblings because we do so well at school yet he doesn't even try and he gets so much praise for anything he does. He got caught shoplifting in TOYMASTER, stealing a transformer for his friend, he cried all the way home and said he would change but i know that he takes scissors with him when he goes shopping to cut the labels of clothing he takes. He didn't get the grades to get to the engineering sixth form he wanted but in the end they accepted despite his bad grades though he does have to retake english language. My parents thought that when they accepted him that he would change but he hasn't, he's just got worse :/. I cry about it all the time, I'm embarrassed to call him my brother. I just wish he'd realize what a freak he's being and grow up. If it was the reverse situation how would he feel? I just can't revise or anything knowing what a failure he is and he's bringing me down. He always puts me down and is unnecessarily mean to me. I always snap at him too and my parents because they just think he learns but he doesn't. I know this isn't about me but I don't know what to do? Can I just sit back and let him throw away his life? If I do, it'll be us, his family who has to pick up the pieces. He wants to go to university as well, he thinks its realistically possible but everyone else can see its not but they just don't want to crush his independence.