well you kinda have to give his res back ( since he is the owner of it) keep trying to let you keep it for now but if not just take all the resources your brother left there ( tear down the castle)
well he can't get perma-banned for wanting his res back just take what you can from it before he resets your flags
I'd basically get your stuff off that res ASAP if I was you. There's nothing stopping the 'friend' from removing your perms. It may feel harsh, but at least you get the enjoyment of building a res again.
this should have been a sign not to build a overly lavish project on land that does not belong to yourself. As others have said, this is not your res and as such you have no claim to it, I suggest calmly talking with the owner in a Private Message and not by slandering the name on the Public forum for all to see.