Put a solar powered MeshCore repeater on top of your house & give a MeshCore companions/clients to your family members.

https://en.wikipedia.org/wiki/MeshCore

https://meshcore.co.uk/

https://meshcore.co.uk/map.html

Your family memebers should be able to send you encrypted text messages using the MeshCore network only, with no dependence on cellular network or the Internet.

They just need to be in range of your repeater or in areas covered by other nodes that can transitively reach yours. If you are too far from other repeaters to connect, you might need to arrange for some intermediate repeaters being installed. But due to their solar nature (with solar charged battery for night operation), only a secure mounting is needed - this makes gaining permission of their installation a lot easier. With good placement a repeater has a range in dozens of kilometers.

Some example hardware currently recommended for use:

client/companion: Meshtastic Nordic nRF52840 SX1262 LoRaWAN LoRa Arduino Positioning Devboard Low Power TFT Display BLE WiFi Mesh node T114 V2.0" - https://www.aliexpress.com/item/1005007593391057.html

solar repeater: SenseCAP Solar Node P1-Pro for Meshtastic - https://www.seeedstudio.com/SenseCAP-Solar-Node-P1-Pro-for-M...