You already have your family using an encrypted service WhatsApp. Better to make them verify encryption keys with each other. And set group to auto delete messages after 30 days.
We hate what’s app because of all the metadata leakage. But it’s metadata of your family being a family. “They” already know you are a family.
If you can cinch the win though, then get them onto signal. If not, harden WhatsApp for the whole group by showing them how to use settings.
OP mentions that his family already uses WhatsApp. It’s a far bigger jump from Skypes encryption. I’m telling them to not be discouraged because they’ve already gotten pretty far.
I don’t think their normie family is going to be dragged any further than corporate end-end encryption, with dedicated servers in every country. But I like being pleasantly surprised.
And frankly, no government is going to be able to hack meeting up with your family for a face to face talk of sensitive info.