If you’re happy with OpenDNS logging your traffic, you could set the router to use them as the DNS server (assuming your network is DHCP and your father-in-law’s devices just use whatever DNS server your router says). As long as one of your devices keeps OpenDNS updated with your IP (the one provided by your ISP), then you can look through the traffic to see what is being accessed (and even enabling category content blocking if you wish). Of course, if your father-in-law is IT savvy, he’ll just manually set the DNS server on his devices.