mac2mac LAN sharing without WLAN, is this possible?!

1st if you have a router, then the router is a firewall preventing your neighbors from seeing anything connected inside your home, assuming you are not using WiFi without any encryption, or using the very weak WEP encryption, and assuming you have not done any port forwarding on your router for port 548 from the internet to one of your systems, and assuming you have not put one of your Macs in the router's DMZ or made your Mac the default receiver for all unsolicited internet network connections (note this requires you take extra setup steps on your router, and that your router actually have these features).

If you direct connect 2 Macs with a single ethernet cable, turn off WiFi, do not attach to your broadband modem or router, then your systems will not be seen by anyone except the 2 Macs.