No need to spell out this reaction: big sis is overjoyed
A little girl is about to become a big sister. But she doesn’t know it yet.
In order to receive the good news, she must first spell out the words on a T-shirt given to her by her proud parents.
It’s a little slow going (remember when you first learned how to read?) but eventually she sounds out the words “I’m the big sister” and the meaning sinks in.
Her reaction starting around the 3:35 mark is the big payoff.