I think your idea of a Island may have merit. Except I would not suggest dropping all the drugs in the middle of the island. You would end up with dead people from over dosing or killing each other for the drugs. 
My suggestion would be to have all of them get put on Suboxone (for opioid addicts) and have them start on it before they get to the island. I have no idea if it works for meth or other drugs, it might because what it does is to block the part of the brain where the cravings are. I think a community away from everything except the basic necessities of life where they have to build, grow, fish, and gather would work for those who have seriously decided they want to live. My opinion is that most addicts are committing a slow suicide and as they do, they put their loved ones through hell as well as others in the communities. They have to decide that they want to live and that means getting off the drugs and alcohol. I am sorry for sister, she is very lucky to have you!