yes, you can do that. All that you need to do, it's change IP in client-side grf with GRF editor to your IP of your local one IP (192.168.0.* or I dunno which one are you using).
So everything is simple, for example, you have 2 PCs, PC A (yours) and PC B (your brother)
Both of you connected to lan and you see each other PC's LAN IP
For example, your IP is: 192.168.0.5, mask: 255.255.255.0, gateway (your router one, or no gateway if you connected directly via cable without any router)
So, you need instal Server Database Editor and edit clientinfo.xml and change 127.0.0.1 to your 192.168.0.5 IP address (just example numbers, make sure you put here correct one). Save file, save grf. Try to run the server, then run the game client. If it works, copy-paste your game-client to your brother PC, it should work