Failure to Thrive?

I began fostering a pregnant stray about a month ago. She gave birth two weeks ago to four adorable babies and promptly rejected them. So I took on bottle-feeding them and stimulating them to void. They just turned two weeks and were doing great I thought. I've fostered puppies before but never kittens.

But last night they started getting a little lethargic and not wanting to eat as much as usual (but still eating a few mls at every feeding). They were still peeing and pooping, no vomiting, no diarrhea. They were sleepy, but would wake up, begin mewling, and crawling around if I touched them or made noise. They were maybe a little cool to the touch, but I had a heater in their room so I didn't think they were chilled.

Then this morning I came in to feed them three hours after the last feeding and two were deceased. I rushed the two others to the emergency vet, who said they seemed stable and sent us home with prophylactic antibiotics. He didn't think they had parvo or feline leukemia since they didn't have any other symptoms. He said cause of death was likely something congenital, or possibly silent aspiration pneumonia, or just plain failure to thrive.

I can't afford a necropsy, so I won't be able to find out for sure. I'm so devastated. Please tell me if there is anything else I should do to protect these two remaining babies! And any encouraging words would be appreciated as well.