It’s a fake layered photoshop composition.
When you invert (negative) the image the edited areas become more noticeable.
1/. Kate’s head, hair, hands and torso have been layered in.
2/. Charlotte’s hair has been layered in around the shoulders and above Kate’s left hand.
3/.George’s hand have been layered in
4/. Kate’s hand on Louis side has been layered in.