When this picture was released, Colette was accused of taking the whole mouth and smile from one of the girls rather than blending them together. He didn't, though. They have identical smiles and teeth, but if you look closely, their lips are slightly different.
Veronique is older than her sister Catherine by six years, and we think that age difference is just-about visible in this image, but barely. If Veronique had worn her hair in front of her ear for the photo, it might have been impossible to tell them apart.