This may be unpopular? But Signal > iMessage > WhatsApp, is my personal ranking. Usually it’s possible to get almost anyone on one of those, and while WhatsApp is far from perfect, it’s better than SMS or another totally unencrypted service.

As a certified Meta hater, I would love a better third option, if anyone knows of one, but WhatsApp has such strong network effects, and it seems like a decent way to moderately* improve the security posture of non-tech-savvy family.

*The thing where they store the encryption key with the backup is terrible, and can make the improvement essentially negligible in cases of govt subpoena or similar, but even so, using WhatsApp would still prevent stuff like this SnapChat story