
What makes you think the little girl might be fading or is it just the fear after what happened to the boy?
If the girl is gaining weight and seems very strong, I would keep an close eye on her and study what can be done if she does start getting unwell so I would be prepared to tell the vet what I would like done.
I would stop taking her to different vets as this is a stress to her/her mum and also possibly getting exposed to bugs whilst at the vets.
Is it a possibility the vet took the temperature incorrectly when they recorded that very low temperature? Many vets really do not know much about tiny pups...
Very young puppies do have a lower temperature than adults do and of course are not able to hold their body temperature very well.
I would use a heat mat to ensure she is nice & warm and possibly enclosed pen, weight her twice a day to make sure she is gaining ( make sure she has space to get off the heat mat if too warm).
Good luck and crossing fingers she will be fine