I did this using a router to handle traffic for about a year between sites 240 miles apart. The only downside was it used a lot of bandwidth. Maybe it can be done with some IP cameras, but I expect they'd be expensive commercial models. OpenWRT compatible routers can be dirt cheap.

If your MIL's router supports OpenWRT or you can replace it with one that does, you can use a VPN and PBR (Policy Based Routing) to your home router or server and do exactly what you want with any camera that supports RSTP (Real Time Streaming Protocol). That includes most cheap wired TP-Link cameras and plenty of others. PBR on the router at your MIL's home would be used to route all traffic from camera IP addresses to your server.

I used Frigate software and Home Assistant for occupancy detection/alerts and Wireguard for transport. It worked perfectly but I eventually got around to adding 2nd server.