Just my thoughts on it:
I speak from going through something very similar with my older brother. I personally think cannabis affects different people in different ways and although many users of it defend it, I know many who have said they were fine until they started smoking it and then their behaviour and personality changed to someone much darker. Fine during the actual smoking of it as like people mentioned, you aren't violent when you smoke it, but the day after they'd be moody and irritable and downright strange. After a few years, it really seemed to affect that person. Personally I think this happpened to my brother.
Our relationship seemed to be similar to yours, it wasn't great and a few times we had full blown arguments and fights that lead to him leaving the house for a while.
Realising I was going to Japan and not going to see him for a year I thought it best to talk about it. And so we managed to convince him to come back for a short talk, we said "each person gets 20 minutes to say what they want, what they think and no interrupting". It kinda worked. There was a lot of awkwardness, a lot of stuff nobody wanted to hear coming out. He talked a lot about how I made him feel, and I spoke in turn for how I felt he made home a hard place to be. But in the end as the summer went on, we managed to keep making an effort to talk and as much as my relationship with my brother will never be the best, i can now happily say for the first time in years its a positive one (for now).
I know it's hard to actually start something like that but maybe your parents can initiate it. I think my mum just went for the simple "You know we all need to talk don't you?" in the parent kind of way. It's uncomfortable but there's no way the relationship will get better, nor will anything change for your brother unless you go back to the beginning and see how it began to deteriorate.
Oh and make sure you don't ever use any language like "you have a problem", "you changed" etc. Every sentence needs to start with "For me," "Personally" "I'm not saying you mean to act like this but I find you..." etc.
Please let me know how, if anything happens, it goes. This is a topic that I find hits very close to home.