Tallima: You will need unique CD keys to play. So your brother will need his own GOG account to play -- even on LAN. That goes for NWN1 and 2.
CarSpy: Okay thats shame :S kinda destoys the Whole DRM free thing but oh well thanks for letting me know
That one's not DRM. The game needs a unique identifier for the online capabilities. It just happens to work just as well as DRM for multiplayer gaming.
If you're scripting a digital world, you often want to be able to put your finger on a specific CD key, not Gamespy account or character name. Mostly, it's used for the online worlds to use their ban hammer, but I'm sure it's used to link different worlds together and whatnot.
Anyway, it does suck. I wish it wasn't so.
If you get NWN 1, I hear you can install 1 copy with the default CD key and another copy with the CD key that you request.
NWN 2 comes with just 1 unique CD key, however, so that method won't work.
GOG certainly frowns on sharing your games freely (DRM-free doesn't mean "please steal me") but I think they don't have problems if you put it on a few computers at your own home. If your bro plays one and you play another, it would legally be fine (I think). But you won't be able to get past that CD key issue.