Valve is on record that they will unlock your games in this unlikely event.lapan said:The only big problem i see with steam is that you don't own a physical copy of the game anymore. Should Steam ever be closed down, there is a high possibility you will never be able to play/reinstall your games.
Anyway, offline mode was the answer so now everyone (with a decent internet connection) can love Steam, yay!
BTW OP Once you have setup your Steam account on your brother's PC, Steam will want to download the games when you install them. You can force Steam to install from the retail copy, but I suspect you will need to be online at the install stage. Save yourself some bandwidth.