I did the same thing as in the office - embarrassed or annoyed them (in a small way) by using their lack of security. I changed desktop backgrounds, "stole" £20, sent emails with promises like "I'll wash your car" to people. I'd follow this up with a lecture on "if I can do this, imagine what some dodgy foreign hacker could do".

Constructively, I pay for the whole family's 1Password and Fastmail accounts. I am the admin. I'm patient and understanding when they do something wrong. And I limit the number of people I help to those I can really help.

We have a WhatsApp group where they can ask whether something is dodgy. They don't use it for chitchat, so anything that comes through, I treat urgently.