> * Opensource (server & client)
Can install your own prosody instance or use one of the many free xmpp servers avalable.
> * Available on Linux, Windows, iOS and Android
I don't know about iOS, but on Android Conversations is excellent: https://f-droid.org/packages/eu.siacs.conversations/ and https://play.google.com/store/apps/details?id=eu.siacs.conve...
1. Set up private test room on riot.im/matrix.org server.
2. Have family members download mobile riot.im app/client.
3. Have them play around with the system - specifically within the private room that you set up - and see what they like/dislike.
4. If they like it, and you aren't too worried about privacy, keep using that private room on the riot.im/matrix.org server.
5. Alternatively, for maximum privacy, set up your own matrix instance - using the reference synapse server - on a private VPS that you control, etc. Because of federation (for the win!), you can always later connect with other servers/rooms (that are not on your server, think like email federation).
Honestly, the UI on riot.im is so very intuitive, and has a very low learning curve; quite newbie-friendly (in my opinion). Good luck!
* Simple for family members to download and use?
* Opensource (server & client)
* Available on Linux, Windows, iOS and Android
Riot/matrix seems like a great match, but I worry that family members will be unable to use it easily.