How do you convince someone that you catch more flies with honey than with vinegar?

When you really need to manipulate someone, for the welfare of another person, like an elderly parent in the grip of an uncaring control freak, or a baby in the grip of an uncaring, but control freak mother, how do you convince someone that getting in the control freak’s face, or raising six kinds of hell all around about that person is NOT the way to protect the elderly parent or the baby?
Take the case of a mother who doesn’t really care about the child, but knows she can use it as a pawn to control others around her. How do you convince someone that it’s important to be that mother’s ‘friend,’ even if you can’t stand them, in order to have the best chance to protect the baby?
How can you convince them??