S.H.
I haven't had this problem, but do you know what is the root of his wanting to sleep with his brother? Does he want to keep his brother safe? Is he scared himself? Is it because that's what he's used to? Does he need his brother next to him or something to make him feel secure?
I would talk with him a bit and find out. If it's just that he wants something to make him feel secure, what about making a "brother" pillow buddy (doll?) or something like that? :) He could help you make it. I have not made one, just thought of it now. lol I would supposed that you could make one out of a pillowcase and stuffing or pillow and decorate it with fabric markers and a face and maybe even put some of younger brother's old pajamas on it. lol