You need to make sure that you are (1) fully patched to the most current version or at least the version the LAN host is using, and (2) you have any enabled mods or custom maps that are being used enabled and installed.

To upgrade to the latest version of Sins, you need to download and isntall Impulse, which is Stardock's version of Steam (but perhaps much more benign) and the form of DRM used for the game (pirates can't use the new patches because they need a legitimate CD in order to patch it up over Impulse).