Bitmessage, Tox, Wire, Matrix. Even Signal offers a desktop app, but it needs a phone number to register.
Perhaps you're referring to the fact that there's no 'standard' solution. This is the way of things, it seems - network effects weaken as choice increases.
Moxie Marlinspike has commented that Wire does not use the Signal Protocol but only took some components from it and created their own protocol, which he did "not recommend"
Has there been any changes to that situation?
* Totally multi platform, even for platforms not yet invented
* Totally robust, though you might want to follow an operational guide for this 
> The primary problem with PGP is that there is no Forward Secrecy. Losing a key means that all content encrypted with thatkey is compromised. There are two ways of dealing with this:
> create a single master holy grail key and guard it with your life
> create keys frequently and destroy them as soon as they are no longer needed
You can pair it with conversations on android (https://f-droid.org/packages/eu.siacs.conversations/).
signal and whatsapp’s “native clients” are chromium-driven, but can run independently of the browser.
* 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.
> * 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!
Any mail client with either PGP or SMIME.
I still don't trust their encryption when compared to Matrix.