Yes, it should be. Check your multiplayer settings in the Game section of the options; there are separate options to enable visibility online (Steam, and eventually GOG Galaxy I assume) LAN and Direct Connections.
As long as you have LAN enabled, you just have to start a multiplayer game, and your fiancee click on Join from the main menu (switch LAN in the lobby screen, if required).
If Steam disconnection still cause a problem if you join via LAN, start the game directly from the '..\SteamApps\common\Divinity - Original Sin\Shipping\EoCApp.exe' program file, and you don't need to have the Steam client running.
