Nagios Support Forum

Not really sure what to look for in the docs so I figured it's quicker to ask.

I have a situation where I want to monitor the computers/voip/routers at my Parents house. But I can only get to their main computer. Not to their entire network via vpn. So I want to run another nagios instance at their house. Then have the results on my screen.

Is there a way to link two nagios instances together?

Yes.
http://nagios.sourceforge.net/docs/3_0/distributed.html

However, it might be easier to just use NSCA or NRDP to send passive checks to your Nagios server. That way Nagios doesn't need to get to all of the devices, as long as the devices can send to Nagios.