Have you tried connecting both computers to the router, independently? You may have to set up a new connection (through windows) on your sisters PC as well, bit try it without setting up new connections first.
Sorry, rereading you're post it sounds like you already have it set up that way?
Just to be sure, this is how you should have it set up:
Also if you have internet connection sharing on your PC, that might be causing a problem.
Edited by jhsmurray, 04 August 2007 - 05:13 AM.