I convinced my family to try Signal, and after a month of not being able to connect despite knowing each other's numbers - silent errors - I had to apologize and join them on WhatsApp.
They all use iMessage primarily, but that's a whole other can-of-worms conversation. (Screw Apple.)
Got my entire family on Signals (sisters, brothers, in-laws, parents, nephews…). We're very happy but could extend that to second degrees(?), to cousins and uncles. We're were close to succeed but we needed help (media, mostly).