Can i ask where you are in the world? And what do you mean by 'foreign breeder'?
FIP is not infectious. It is a mutated version of 'Corona Virus'. Did your vet ever aspirate liquid from your kittens abdomen? Your kitten could show high titre levels for Coronavirus and not have FIP.
Your second kitten left the breeder far too young. She too will probably be positive for Coronavirus but will not necessarily develop FIP. Is she related to the boy?
Can i ask if you keep her caged? I notice a cage in your pictures of her similar to the cages the breeder had. I hate to see breeders using these cages, notice how the poor mom paces in the video you posted. It is an irresponsible breeder that breeds in this way choosing money over the health and welfare of their cats. Please don't use a cage to confine/keep your kitten in. It is cruel and unfair on them