She had her vaccines too soon so the antibodies she got from nursing blocked the effectiveness of the vaccine. She shouldn't have gotten her first vaccination until several weeks after weaning and her last should have been between 16-18 weeks.
In one study of a cross section of different puppies the age at which they were able to respond to a vaccine and develop protection covered a wide period of time. At six weeks of age, 25% of the puppies could be immunized. At 9 weeks of age, 40% of the puppies were able to respond to the vaccine. The number increased to 60% by 16 weeks, and by 18 weeks of age, 95% of the puppies could be immunized. Parvovirus: Serious Diarrhea in Puppies & Dogs
Her littermate is probably at a high risk of getting it also.