I had the smae problem with my 3yr old. I found that persitance paid. When she would climb into my bed in the middle of the night I would get up and put her back in her own room. It took a few nights, but it finally worked. I would suggest you try the same. Be strong , I know its hard to hear him cry. GOOD LUCK!!!
It would be better for him to be in another room as you say his brother may mean well but it could be exacerbating the problem,and if he moans at him that wont make it better . The only way really is to keep putting him back into his own bed,even if he yells, just ignore it ,he knows that you relent and eventually go in to him ,so he gets mixed messages.and he also knows if he keeps it up he will end up in Moms bed.,so why would he co-operate when he doesnt have to.?