If it was me in the same situation, I absolutely would not want to be in charge of a trust for a sibling that has a serious issue with drugs/alcohol.
It is at best going to cause you to have to deal with someone that is irrational every time they want more than what is scheduled.
At worst, you are opening yourself up to a chance of harassment, threatening of bodily harm to yourself or you family, and other violent outbursts not just from them, but from their scummy friends/acquaintances. (like those "debt collectors")
I would tell the parents it's a pass as you are afraid of the issues that it would cause for you and him if he had to ask you for permission to spend "his" money - he could end up hating you even if he sort of got why it had to be that way. It would likely prevent you from ever having a decent relationship with him even if someday he cleaned up his act, so I'd push that as the main reason rather than being scared of him and what could happen if he went a bit "off" or got angry or whatever.
And whomever they get, do ask for them to please make sure it's someone in authority that can stand up to a manipulative person/deal with addict. They'd need to make sure and get professional advice about setting up a trust to deal with a person like this, but honestly your parents should not leave him anything unless it is to be used for rehab and paying of bills directly, never giving him cash directly if possible, but that's strictly my opinion and I'm not even sure this is possible.
So sorry your family is dealing with this. Addiction is a terrible thing. I hope your brother gets help someday before it is too late.