Or maybe it's "I love you" in American Sign Language. I know that was the first and only ASL sign I learned when I was a kid, besides how to sign my first name. They taught it to us in school.
Or maybe her dad just pulled her hand out of her mouth seconds before the camera flashed (but missed the younger sister, hah).
Sorry, my three youngest are 13 months apart (Irish triplets), trying to get a photo of all three of them smiling at the camera photogenically simultaneously is why photoshop was invented.