How do I convince household members to improve their oral care habits or diet in reaction to having a cavity?

Frustrating isn't it. You can talk wheedle, cajole, show pictures, rant and rave, ask dentist for help, but nothin's going to work as long as your family members feel entitled. They think they can go out into a lightning storm with impunity. They won, t wake up until actually struck by lightning. Ask your dentist to try to educate your family, but don't expect much if they don't want to learn.