The young princess also has another reason to be excited: She's making history as the first female royal to retain her claim to the throne regardless of if her new sibling is a boy or girl.
The two youngsters were on the way to meet their new little brother, accompanied by their father Prince William, when the sweet moment was captured. As the Duchess of Cambridge once said, her daughter is the "one in charge". The Duke of Cambridge was present for the birth.
You probably saw Princess Charlotte reach peak cuteness with her wave to the world's cameras - but you might have missed the adorable moment she shared with her older brother just afterwards.
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As the new baby's royal siblings arrived to meet him for the first time, Prince George left the vehicle looking visibly nervous but his little sister Princess Charlotte was beaming and gave everyone outside the hospital a wave.
Having had a baby, and probably not much sleep - she was in labour by dawn - she looked just as serene and together as she did on the same steps after the birth of George five years ago and of Charlotte three years ago. But Charlotte has always displayed confidence beyond her age.
"That is definitely Prince George FLICKING the back of Princess Charlotte's head as they enter the Lindo Wing", one amused Twitter user wrote.
Few young children are as well dressed as those in the royal family. Years ago, the arrival of a new prince would mean he'd frog-leap over his older sister in the line of succession to the throne.