
I had heard of this happening with two sisters one who had a litter in a primitive Spitz breed but this was in pups before they were mobile.
I can't imagine a bitch simply killing such a mature litter out of the blue if she hadn't shown any signs of antagonism, though have heard of Terriers killing theier own pups when they got to the running about stage mistaking their movements and noise of prey, and was quite shocked.