This puppy is half the size of her sister and was previously very active and loud, but now has stopped feeding and is breathing funny. She is just laying on her side and is not even trying to feed. What could be wrong?

Sounds like a respiratory issue, but there are too many possibilities to guess at. You need to take pup to the vet asap. Pups can go from bad to worse to death in a matter of hours. Most vets have an after hours emergency on call vet as well. Please get her help asap.