ya, start with Gamite's suggestion. looks like your using your brothers system as a gateway (via software).
do an ipconfig (via DOS) on the system that works. then do the same to yours.
both should have the same subnet, and a unique private IP address either 192.168.x.x or 10.10.x.x
make sure both have the same gateway.
then through DOS, ping the main system. if you don't get a reply back, then the problem could be your NIC not being assigned a proper IP.
start with that and let us know.