Despite popular belief, sheep are not stupid. They’re prey animals, driven by a powerful instinct to stay with their flock. It’s a matter of self-preservation: Where one sheep goes, they all go. Shepherds learned this centuries ago, and began attaching a bell to one of the sheep in their flock. This allowed them to snatch a few precious hours of sleep, knowing that if they followed the sound of that single bell they would inevitably find the whole flock. That bell-bedecked ovine became known as the bellwether.
In my family, my younger sister is the bellwether. Not necessarily the leader of our actions (each of us desire that role - family trip planning is a nightmare), but the leader of our collective emotional state.