You had the puppies shipped to Florida, right? Florida has some of the toughest puppy lemon laws in the country. You have the right to keep the puppy and get reimbursed for vet bills up to the price of the puppy.
Here is a summary:
Florida Pet Lemon Law
If your sister decides to keep this puppy, please make sure she gets a bile acids test done right at 20 weeks. I still wouldn't rule out a liver shunt.