There is a firmware update for D-Link routers enabeling a sort of
gaming mode for the router.
[ a link to it is in
this topic. ]
Have you tried playing with your computer and the computer on the other side of the router as server ?
Think it might work if her brother is hosting.
Every game that set's up a server to make host an internet game like Unreal Tournament require you to forward a port to it from our router. Others who connect to a online server, like Command & Conquer does might require port forwarding or could work without any problems.
Have you tried placing both computers in a DMZ at your routers configuration ?