Your parents need to really and fully understand the seriousness of the situation.
Maybe I'm reading too much into your OP, but from what you wrote it could be that at least your mother is feeling some guilt because of his chronic disease and thus acting wrongly despite knowing better.
I'm reading this into your words as your story reminds me of this brother of a close friend of mine who somehow managed - as strange as it sounds - to make his mother feel guilty about his acquired type 1 diabetes when he was a child and teenager. Today he's an adult in his early 30s without a job or any perspective (I know the family since my childhoood). Throughout the past 15 years or so he was not willing to make even the slightest effort to get a job or change the situation, he just pretended to do so. Unlike your brother he was never addicted to anything but he lied to his family every single day (the most important example which really shocked me when I heard of it: He left the family for more than 5 years to study law. When he returned he pretended to apply for jobs - over the years he had became an expert liar instead of an expert lawyer - and it took another year until the family realized that he'd actually never finished his studies. He probably never even really started. He was about 25 years at that time and until today he never had a job in his life). He has one younger brother and two younger sisters, and then there's this youngest brother who died in a car accident at the age of 18 (which imo is a key element of this particular family story). His siblings gave up fighting against his egoism and against their mother's will a few years ago. Some day they just had to realize that their mother will always let him do whatever he wants, i.e. nothing, and that their father is too good-natured - and imo unfortunately also much too weak - to change the situation. The parents never really understood or never wanted to realize that they needed outside help. I'm afraid the guy will live jobless and with his parents until they die.
Sorry for the long story. It wasn't by any means meant to discourage you - in fact I think it's great that, unlike your parents, you realized the seriousness of the situation early enough - but, in short and in my opinion based on my experience with this family:
As long as your parents are not in the same boat as you, I don't think you have a chance to change anything.
