As many have said here, your sister is a complete idiot and is actually a back yard breeder hoping to make a quick buck. Basically, she's trying to sell un-vaccinated, mongrel puppies for £300+ without health checks, probably without a decent worming programme and certainly without giving a shit for the welfare of the pups and where they end up - if she did give a shit, she wouldn't have bred in the first place.
I'm betting that she also bred litters back to back.
She needs to approach reputable rescues and beg them for help (they may not be able to, as they are stuffed to the gunnels with dumped dogs and pups that other back yard morons have bred).
She needs to make sure the puppies are vaccinated, wormed, micro-chipped and health checked by a vet, as that will make them more appealing to a rescue. Then she needs to get together a bloody great donation, and give that to the rescue too.
I would suggest that then that when the puppies are gone, the very first thing she does is get ALL the dogs neutered and doesn't get any more entire dogs.
Yes, accidents do happen - I've seen it happen with responsible sheep-dog training & breeding kennels, where there are 25+ entire bitches & dogs, but there is NO excuse for the pregnancy to continue if an accident happens -you visit the vet, take vet advice and get a job to end the pregnancy.
Your sister is a fuckwit.