
Achondroplastic dwarfism is known in a lot of breeds, and of course is more likely to be expressed in an inbred pup as both parents have a higher chance of carrying the same recessive traits.
On the other hand this trait can occur quite randomly. A well known stud dog in our breed in one litter for his owner produced a bitch pup who had legs about half the length they should have been, they were perfectly staight too, they weren't as short as those of a Basset (but then again none of those low breeds had such short legs in the past).
She used to joke that after 30+ years of careful breeding, and the pillar of her achievements was 'Dolly'. She had lots of offers to buy her because of her unusual appearance, but flatly refused. There are unscrupulous people who would deliberately to try to bred on such a 'sport' or freak. Wyhe someone in TV who had a Labrador like this (was it Jimmy Hill?) suddenly people wanted Dwarf Labs!!!
Just trying in a long winded fashion to say that inbreding of itself does not produce problems, it just makes any latent bad genes, as well as good more likely to come together in the offspring.
Unfortunately this is even more likley in such haphazardly bred animals as in this instance, as it is unlikly that either original parent had been selected to complement it's mate, and may even have had very obvious undesirable characteristics, which are likley to be even more fixed in the offspring of Son to Mother. If the people were extremely lucky there may not hagve ben any problems in either parent to pass on, or nothing that drastic, but if there were faults in any of them then they are likely to be intensified in these son to mother and brother to sister pups.