I agree with Hena. I'd try not to let him see the others in mum and dad's bed, if possible.
When my LO was this age, he started wanting to sleep with the bedroom door open. Which was fine with me. But it did give me a tool, so that if he was screaming and carrying on I could say "If you want the door open, you must be quiet. This is too noisy." I'm wondering if something similar could work with sharing a room with his brother. I know it's not the same thing, and using big brother as a tool like that may not work, but I was just reminded of this and mthought I'd mention it, as it may be something that motivates him.