I am not the techiest person in the world, but I know Micro$oft has ways to prevent people from just copying their software willy nilly.
What I think happens is that your copy has a key built into it. When your bros computer connects to internet it will be able to tell that two computers are sharing a key.
Maybe if he never connects to internet.
Other possible problem. I think win7 looks at special ids on various pieces of hardware (Network card, processor etc) and makes something like a fingerprint of your computer. It will immediately be able to tell its on a different computer. Also knowing you ripped of Microsoft will slowly eat away at you.