I managed to get my immediate family (and my sister's husband) onto Threema, the extended family is on Whatsapp with zero chance of switching away.
On the other hand, one cousin's husband can't stop spamming the group chat with pics of their daughters. Not just "special event, here's a pic of each at their best" or whatever, but multiple. fotos. daily., sometimes videos too... As a consequence, I've permanently muted that whatsapp group and only occasionally take a peek at it.