Are your computers both fast enough? I can only host games on my wife's computer (which is faster than mine but has a worse graphics card) because mine chokes on the multiplayer processing.
If you want to get technical about it, you can download Wireshark
to view what's going on behind the scenes.
Wireshark is relatively easy to use (though it is technical). You 'attach' it to your wireless connection and start recording, and you can filter packets by packet type and IP address. Run DC in a window, start a gaming session, and check that you're getting consistent packets from your brother's IP without any issues. He can do the same for your IP. If you see any lost packets (which will be highlighted) that could be the source of the problem.