How can I help my brother through this?

You're in a tough position, because you see what's wrong, but you don't have the "standing" in your home to fix it That's because you're not in a position to require certain things and then impose consequences when he doesn't comply.

Obviously, the point I'm getting at is your parents are failing him. For example, you don't say why therapy was considered, but if your parents think he needs it, it shouldn't be offered as a suggestion! It should be mandated as a requirement for being allowed to live with them.

You don't say much about how he spends his free time, but your parents have to start requiring more of him. It's kind of late in the game, but the fact that he's now 18 doesn't change anything. Your parents can set any rules they want for anyone living in their home.

So maybe the best way to help him is to talk to your parents using "tough love", because they do need to start making it harder for him to slack like this. Just as one example, if he has driving privs, these should be taken away until he's done X or Y. He needs motivation and a kick in the butt if he's ever going to be happy. Another option is call the cops if he's high . Regardless of the weed laws in your state, it's never legal for an 18 year old to be using.

The other thing is lead by example. Don't sit him down and tell him what to do, but be more indirect. If you're doing something fun that makes you happy, whether cooking or shooting hoops, casually mention how important it is to have passions in life.

turn him in, let him spend some time in juvi and he'll learn really fast that its not cool to do that

It's not up to you to direct your brother's life. He will do what he will do... and he IS living in your parents' home. As parents, we create our own monsters....

I don't think that him smoking weed is the end of the world. A lot of extremely intelligent people smoke it, and it doesn't stop them from being exceptional. I think that it's possible that he is depending on weed to relax himself for some reason... maybe he really has depression. I think that you should definitely get him some psychological help, because a lot of people with mental illnesses are often seen as 'lazy' because they are misunderstood. Try and get him to open up a bit, and be very patient with him.