I also thought about what happens when I die and did not want my AS to have those assets. I did not want to be enabling him from the grave!
What I did in my will was divide assets equally between my 3 children. The two healthy children will get their share straight out. The share that would have gone to my AS instead goes to a treatment center that I named (it is a long-term treatment center that exists on donations and does not charge the addict or family for treatment). Then I took Dave Ramsey's advice and told my sons what I had done - so there would be no surprises and to minimize sibling jealousy or guilt if they were to find out about it after I am gone. But I have to be honest here and say that I have not told the AS this yet. If and when he finds long-term recovery and I feel confident about that, I will be changing that will. By the way, I have spent very little money on this AS of mine - probably under $2000. So my decision was not based on the fact that he got his inheritance already - it is just based on my desire not to enable my AS from the grave.
A word of caution on involving your addict's siblings in the whole thing (as in being in charge of a trust and deciding when the addict is "deserving" of the release of assets). That pits sibling against sibling. How could it not?
Good for you for thinking about this whole thing long-term. It is healthy for you and healthy for you beloved addict and healthy for the siblings! Everybody wins!