I wasn’t there when it happened, but the story I heard is that the kids were riding in the car. The younger said “Baby Hungry”. The older looked at her mouth and scolded, “you’re not hungry. I can tell by your mouth. If you were hungry your tongue would be more red!”
Mommy, naturally, thought this was weird. When she told me the story I explained how I had heard that Parent Birds know which chick to feed because the hungry ones have a redder mouth. I understand the blood flows to the digestive parts after they’re fed, so the red-mouthed baby gets more food. The same rules must apply to Little Sister.