Either you or your sister can be the "Server" or if you have another computer that just does not meet the video requirements you can download the standalone server from here
http://nwn.bioware.com/downloads/standaloneserver.html
As andrewas was saying you and your sister would both need a copy of the game but if you go with this version of the server you do not need a CD key for it. The advantage of running another computer as the server is that if one of you wants to play they can just make sure the server is running then connect in and play.
If you are using either your or your sisters computer as the server then to keep things simple just one of you should run as the server. Since NWN is a long game and I doubt you will finish it in a single evening playing. When you finsh for the night whoever is running as the server should save the game. When that person starts again they should go into Multiplayer then Load Internet \ Lan game.
If you want to switch who is running as the server you should copy the save game file to whoever is running as the server right then. You would do this if you are running as the server but your sister has some time and wants to keep on playing then you would copy the file over to her computer and she would go into Multiplayer then Load Internet \ Lan game.
I hope this helps. Please ask any questions you have.
Granamere