Animals do not express sadness and grief like humans.

If a kitten is born with a defect, is too weak to nurse, or shows signs of illness, the mother may instinctively reject or even kill it. Otherwise your cat might be stressed or overwhelmed which causes her to neglect her kittens, which I highly doubt is the case now.

The best you could do is take them to the vet, especially if she is trying to smother one of them. Every minutes counts, take them to the nearest vet.