Realistically though, you could still use a VPN and have it be pretty easy for your family members IF you have access to their router console and IF said router supports network wide wireguard or openVPN connections. Having both networks tied in to eachother that way makes it so that nobody ever has to use a VPN client to connect, but still only devices from their network (or yours) will be able to connect.

Realistically this plan dies the moment someone takes their phone outside of the WiFi range. It’s fine in theory, but fails miserably in non-techie real life.