My brother got NV and installed it on his PC to his Steam account, and is letting me install it on my PC to my Steam account so that I can play it when he's away. However, when I went to install it, I got a duplicate product code error. Are you seriously telling me that Bethesda is going to make me buy 2 copies of this game and that because some people are trying to download it for free online, I'm the one that suffers for it? ...Seriously?!
How is this different from every other game on the market. One purchase equals one license, and before you install/purchase the game, you agree to this. While DRM free software may technical allow multiple instances to be installed, it is a violation of the EULA on a majority of software. If you plan you activate this game on multiple Steam accounts you'll need to purchase multiple licenses.
As stated if you use your brothers' Steam account then you may be able to play the game. However if it quite possible that using/sharing a Steam account on multiple computer is a violation of the Steam (client) terms of agreement.