If he is your helper and the owner's brother then my question is: Is he subordinate to you or is he fighting your authority? If he is disregarding your leadership then you should have a sit down with your employer and tell him what the deal is.
I have worked for family businesses before and have had issues with the bosses son. I had talked to my employer at the time and voiced my concerns. My employer addressed these issues at the time but it never ended up getting resolved. That was frustrating but there was nothing else I could do. I endured it for a few years letting it ferment and when I came to my snapping point I left the company knowing they were their own worst enemy and nothing I could do would fix their poor judgment.
That maybe the only thing you can do that is fair to all involved.