Nic and Occy, this is the first time I've seen you in agreement - and rightly so.
Can I quickly add my two cents: Firstly - Mini Foxie is NOT a breed, so these pups are in no way purebred. If they are Tenterfield Terriers they should be registered breeding stock. If not, they should be desexed. Likewise if they are Fox Terriers.
Secondly, yes accidents can happen with dogs, but this can only be between registered breeding animals of the same or different breeds as everything else should be desexed.
And if a registered breeder has an accidental litter, the resultant pups should be reared at the breeder's expense (if the pregnancy continues that is...), rehomed responsibly including all vet work and desexing, at NO profit. Also full breeder support should be part of that equation, including a no exceptions lifetime return policy if anything happens to prevent a new owner from being able to keep that dog.
Breeding is a big responsibility, not a pocket money making venture.
I hope your sister takes note of such ethical ideals and considers the lifetime welfare of her own and any future dogs in her family.