I don't think that OP's sister should get any dog, if she can possibly be talked out of it. This is a living creature we are talking about.
Many people are suggesting a miniature pinscher because they look like tiny dobie - but these are highly intelligent, energetic little dogs with strong personalities which can also become both people and dog aggressive if not properly trained and socialised - they are also often resource aggressive. Okay, the chances of them killing someone are pretty remote, but they could badly scar a child - and dog bites, even minor ones are bloody painful!
I think that OP should try to contact the breeder and hopefully they will not only be ethical about it, and not sell her a puppy, but will warn other breeders in the area. She should also try again to talk sense into her mother - point out the size and power of her own dogs and say "If one of them attacked you - what sort of a chance would you have?"
Of course, the likelihood is that the sister would lose interest in this poor dog long before it reached full maturity - and then it would be dumped in a rescue for someone else to sort out its problems, or would be euthanised - or worse, sold on/re-homed with no warning given to the new owners, who might have children or other pets that the dog might attack.
Or maybe pressure would be put on the OP to take the dog her sister no longer wants, and she either does, and has to deal with all of its bad habits, or refuses, and her mother and sister blame her for whatever happens to the dog.
OP - I really don't envy you your task. Is there a dobermans club anywhere near you, where you could take your sister, and perhaps other people's opinions, and the sight of all of these huge (terrifying) dogs would bring her to her senses.
This is so sad.