OpenVPn is free and there are many clients for different platforms. Generally there is a client key (which you create during setting app the server) that you send to your mom. And she uses that key in the client app to access your pc ip securely, though she still needs to know your ip.
Openvpn is a server app and free you do not pay to buy it or run it somewhere. It has to run on your Calibre pc.
https://www.youtube.com/results?sear...envpn+tutorial
Basically Vpn is a secure tunnel between the client and the server. It can be used for many things including accessing server drives etc but it is secure since you need a key(and a pass if you setup) to initiate a tunnel between the client and the server.
Your isp or wifi or 4g provider cant monitor the traffic goes through Vpn if it is setup properly.