Hello 498k, wanted to add my two cents.
My sister is not a coke addict, but she's been on marijuana/alcohol ever since she was a teen. That was thirty years ago.
She has two daughters - they're watching this whole thing go down and I have no idea what is going through their heads. They weren't enough (NOBODY'S enough) to stop my sister from throwing her marriage down the toilet and get involved with another man who is also addicted to pot.
You need to take care of yourself - easier said than done, eh? I've spent a long time as the family fixer-upper. It is a role that is not sustainable in the long run. I used to serve as referee - all that accomplished was a higher sense of entitlement on my sister's part to continue her downward spiral.
I spent a lot of time getting very angry about the lies that she told me to get me on her side. There was a crazy bit of time where I was even thinking about hiring a Private Investigator to catch her in her lies, but then I realized that was utterly pointless. She would still be stuck. She still wouldn't listen.
Many people here have been emphasizing that you need to take care of yourself, and that is absolutely true! Please don't turn out like me, stewing in anger and all that other ugly stuff, because you've invested your time and love trying to save somebody who, at the moment, doesn't want to be saved. Because you will pay the price for that.
I like to use this analogy - your brother is in the ocean and he's drowning. You throw in a life preserver after life preserver and he isn't taking it. You love him so much that you go into the ocean yourself and try to put preserver around him but he shakes it off. You try time and time again and he refuses to take the ring. Selfishness isn't the primary motivator - he's so sick that he most likely can't even make sense of what he's doing. But regardless of the reason, he's pushing the life ring away and taking you with him. After a while, you have to get out of the ocean. You have to take care of yourself, or else you're going to be as lost as he is.
As to the infamous "turning point"? In the end, that is something that will be determined by him and him alone.
I'm so sorry that you're going through this.