I don't doubt that. Surely, they wouldn't rob you. They love you. But your grown-up kids and wife of decades are only humans. Humans make mistakes. The more people you reveal the seed to, the higher the chances the security of your coins is compromised. Period.

Wife has a dear sister, they love each other to bits, and women like to talk a bit too much. Out in the pub, chatting away, wife casually mentions to sister that she's OK financially, because husband (you) is really smart, he has taken care of it, by owning Bitcoin. Wife's sister doesn't know/understand much. A few days later she casually mentions this to her colleagues at work: "maybe we should buy this thing called Bitcoin, see, my sister's family is set for life, they own lots of them." Word spreads. The wrong kind of people hear about it. Next thing you know, your family is kidnapped and the seed is wrenched out of you (or the wife). Coins gone, even lives are in danger. Just by a simple, innocent slip of the tongue.

But sure, I see your point and I agree, it's just that there's no need to overdo it. Just pick one or two very special people in your life and make sure they understand that they should never ever talk about it to anyone, even those very close to them. Then, as already mentioned, there's Shamir's Secret Sharing which is a very good way to secure your coins by splitting the seed to different pieces.