I have no idea if KOTF would be a problem, but I suspect it might be. I tried to stay as far away from that thing as possible :P
Generally you have a cable or DSL modem that connects to the internet, and then from that you connecte to a router. Then each computer/gaming console in the house connects to the router. A router requests one ip from your ISP and then translates each request to and from the computers in your house so that you can have multple things hooked to the internet but only use one IP. It also has firewalls, which block all ports except http, port 80, and whatever else you set it to open up.
Anyway, if your brother is in the same house as you, then you don't have to worry about forwarding any ports. You just start a game server, and then he starts his game and searches for servers making sure to put Local in and NOT Internet servers. Then he should be able to connect.