There are a few frenchie boards out there. Perhaps you would get better answers by inquiring there. Maybe frenchies "grow" out of it, but it would be best to ask experts used to the breed. The sister is the same? Was this the first time this particular cross was done? How have the stud and dam produced in the past? Do they always produce pups that are shy at first and then develop normal frenchie personalities? How has the breeder proven her knowledge and ability with the breed? Showing, conformation, obedience etc? How can she demonstrate the soundness and health of her breeding stock? Some questions that I would consider before getting a dog from this person.
Well behaved, adaptable, and comfortable companions with an affectionate nature and even disposition; generally active, alert, and playful, but not unduly boisterous
No mention of shyness or avoidance behaviors.