I may have posted this in the wrong section as I'm sure newbies do not know how to answer this.

Due to the lack of answers here from the security experts I'm assuming they think I (we) want to hack someone or something I shouldn't. I have my brothers permission and he has mine. As far as ISP TOS are concerned, Were not concerned. We will cross that bridge when we get there. Our ISP connections are our own and if we loose our accounts due to TOS violations then so be it. I see no better way of learning how to pentest a system in a real world enviroment than learning in a real world environment. Of course I'm completely open to suggestion as I may be going about this completely wrong. I do not want to loose my ISP service or be visited by one of the 3-letter people.

On to the question.

My brother and I are playing "Capture the flag" (Hacking each others computer) game(s). We have been doing this through LAN only but we would like to do this over the Internet due to limited time for work and family, also to learn more about pentesting.
So, my question is How would I scan/access his computer using nmap,nessus or whatever I choose since he is behind a router and I am as well.
I only want to access/scan/attack HIS system and not his family's.
Sorry for my cheezy graph below but it should help explain a little better.

|_____My System IP:192.168.100.2 (BackTrack 4 pre-final)
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|___Router (WAN IP: 122.111.123.123 /// LAN IP: 192.168.100.1)
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|_((((The InterNets))))
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|___Router (WAN IP: 18.16.100.5 /// LAN IP: 192.168.100.1)
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|_____(TARGET) His System IP:192.168.100.2 (Windows XP SP1)
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|_______His Wifes System IP: 192.168.100.3
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|_________His Daughters System IP: 192.168.100.4

So to recap, the question is: How to access/scan a computer system that is behind a router?

He has 3 active online systems (all 3 are WinXP) behind a router.
I have nmap'ed his IP which only shows 1 active host running (best guess) Linux so I know I'm only scanning his router.