You can set up what's known as a peer to peer network for wireless. Basically, in peer to peer, two computers with wireless cards connect directly to each other instead of going to a wireless router, like using a crossover cable to connect two computers directly together instead of going through a hub. You'll need to wireless adapters for this, one for your server, and one for your brothers.
I don't know the specifics of how to do it, because I've always gone and bought the access point or router, which would make it a lot easier. Someone also told me that if you do the peer to peer network, the computers have admin rights to each other, ie, your brother is going to be able to see everything on your server, and your server will be able to see everything on your brothers computer. I don't know if that's true, though.
If I was you, I would go pickup a Linksys wireless G router for $80 or $90 american. It will make this easier on you.