I'm guessing you never bothered to ask anyone to explain anything to you in the many conversations you've had over the boards either. This is why you should know It's when stuff like this happens that we need people to be prepare Save the lecture my ass.
Look, I get it. Your brother is important. I've got one, too, and if something tried to possess him I wouldn't even know what to do with myself.
You're gonna need to trap the demon that's possessing your brother first in order to exorcise it. Do you think you can lure him out someplace? I'm willing to help you lay out some markings that'll literally trap him to a certain spot. Then you can exorcise him.