Your brother is in a horrible position. I want to say - fuck his lazy arse brother. But it’s not so simple, I know.
However... what is simple is that your husband was a fool to pander to his brother’s ego by calling him a “manager” and setting him above other staff. No wonder the other staff were unhappy about that!
It sounds like you can afford to carry him, and for your husband - when push comes to shove - wants to carry him.
What I would do differently is take away any pretence of calling him a manager and take away the elevated salary. Pay him enough to cover the rent to you, and what a low skilled worker would get. Adjust it upwards a bit to cover the rent, if you have to. But what he should have left in his pocket to spend should reflect a low wage.
I would avoid even verbally using phrases like “right hand man” which make him sound valuable. He’s not. If you really have NOTHING he can do (that he can do without pissing off other workers) then pay him to do nothing.
Make him wash some farm machinery every day. If he’s shit at washing it (which I suspect he would be) then pick something that isn’t washed daily, usually. Get a proper staff member to wash it on the correct schedule, whilst he washes it daily.
Or don’t make him parts manager, but get him to count the parts and dust them every day, when not needed.
Hell, make him put a chair outside the tied house for 8 hours a day “security” - but please don’t call him the security “manager”.
Give him something easy, dull, and unnecessary - but cut his pay and don’t give him a fancy ego pandering title. Right hand man, my arse!