This is literally the reason trusts were invented: so that you can provide for those who inherit from you when you know they are not in a position to be responsible with a windfall. FIL could still sell the house but only have access to half the cash, and would have monthly income thereafter so that his addiction doesn't render him homeless.

There are not good solutions when it comes to addiction, so everybody should be seeking the least bad option. Putting the husband in this position is a bad option, but having an administered trust pay out monthly ensures that the money goes to what the MIL wants it to go to - making sure that FIL is okay.