To be frank with you, humans are the weakest security point in any system. Even if you did somehow (impossibly) 100% secure your device… you’re literally sending everything to X other family members who don’t care about security anyway and take zero preventative measures. That’s sort of the point of a chat app. All they would need to do is target your family instead of you to get the exact same info - this is how Facebook has everyone’s telephone number and profile photo, even if they don’t have an account. And if it’s a WhatsApp data breach… well. Your family is just one in a sea of millions of potentially better/easier targets.
If there’s anything interesting about your family chats that is actually secret info, it probably shouldn’t be put into text anywhere except maybe a password manager. Just tell them not to send passwords or illegal stuff or security question info via whatsapp. It’s all you can realistically do in situations like this.
We literally cannot keep all information private from everyone all the time, you have to pick and choose your battles. And even then, you’ll still lose some, even if you’re perfect.
Legal advice given to me by an employer treated all citizens as eligible. Their advice tends to err on the side of caution at the best of times, but I have no reason to disagree that it’s at the very least legally contentious even if not yet officially contested.
Tl;dr I wouldn’t want to rely on it in court, whether everyone else is happy to risk that is whatever.