Excuse me if this is off topic, but it seems that you have gotten further than I have. So here goes.....
I have one copy of Sins and am trying to start a LAN game on two computers in my network. I am able to see and create a game on either computer, but when I try to join, I get the following error message "You don't have the files needed for this game." Any ideas on what the problem is or how to fix?
Are your games both the same version? and do you both have the map file on your computers?
If your using a custom map on lan, i had to transfer the custom map file to the other pc first before they could connect, just a few ideas tht might help.