Two PC network with Netzero, one computer can get on internet even when I am on

hey, my sister and I have a network set up between her computer and mine. I use Netzero to get on the internet, and she used to be able to get on AIM on her computer when I'm on the internet on mine. We tried today, and that's not working. Netzero is a shared connection, our IP's are set up correctly, she can ping me fine, I've never been able to ping her because of firewall, we can net send each other, and our link light on both of our NICs are on. I've tried to change her IP, found a Winsock XP Fix program and ran it, checked all connections, and still she can't get on AIM while I am online. We are connecting through a Dlink DI-604 router, and we haven't changed anything that may affect our connectivity. We have also scanned for viruses and spyware and found nothing. Any suggestions?