Have you thought about sending one of the bodies in to be tested? You said this was a "stray" cat, perhaps she had a disease? Unless you asked the vet to test the mother and kittens for all possible diseases, she might not have known there was anything wrong in the first place.
And I REALLY really hate to say this.... but, as others have pointed out, somethings these things happen and there's nothing you can do about it.
I wish you the best of luck with the survivor!!
Annie B. <:3 )~~