What do I do about my girlfriend's brother?

As I’ve mentioned before, my girlfriend’s father took his life after some tough times where he was battling addiction and various mental disorders. She mostly stays at my place, her mother has to work most of the time now, and her brother, Clay, still goes to school. He is just 17 and I’m worried about how he is dealing with his father’s death.

He has turned to mixing xanax and muscle relaxers on a regular basis, sometimes with alcohol. He parties irresponsibly, letting his “friends” tear up the house and eat all of the food while he gets faded. His school work is suffering and he skips a lot because he is always “sick”. He is mean, short-tempered, and quiet most of the time. He spends way too much money on recreational drugs, concerts, gas, food, cigarettes (money given to him by his mom, who is terrible at budgeting). Even his close friends have started to notice that he is having a hard time coping.

Their mom has basically has given up on them except to give them money and put food in the fridge. She offers little guidance and when Clay gets upset she just throws her hands up in a “Why are you doing this to me?” kind of way. She may not see it, but her selfishness is hurting her kids in a bad way. I convinced my girlfriend to talk to her mother about her brother’s xanax problem, but he talked his way out of it and its worse than ever. How do I get this woman to take care of her kids?

I understand that all three of them will have to deal with the suicide of the father in their own way, but I really think its time to do something. My girl has a chance, because she is smart and stays with me away from the family drama. Her brother though, is heading down a really dark road, and I would really like to help. He really likes and respects me, but I don’t know if its my place to talk to him. Their family is currently in crisis, with no real sign of hope. Of course family therapy would be an easy answer, but I doubt they would listen to me. I can’t let this continue, and I know if I could snap their mom out of her daze, she would be the mother they need right now.

Do I let things run their course, or should I intervene? If so, how?