BIL,
My sibs are all drug addicts so I do understand. Three of them lived with our parents until the parents passed away. They were then homeless.
The good news is my brother is now clean and sober and has been for some years. One sister is still struggling but making progress. The other sister I haven't heard from for years.
Believe me, there's nothing you can do to change the situation. Your parents sound like they are fine with it. It's their house, their rules.
My best advice to you is move out. Get your own place. Now *that* would be action. We can't control other people. As unfair as it may seem to you, you need to take care of yourself. Alanon and Alanarc are great suggestions to help *you* move on from the pain and frustration.
I wish you well. I know it's hard but I had to detach from my situation and focus on my own life. I'm an alcoholic in recovery. I couldn't get my own act together until I removed myself from the insanity.
Love,
Lenina