They will line the nest with their belly fluff, for the babies and so their skin makes better contact to heat up the eggs. Are the eggs hot to the touch when you get there? Sounds like if you want you will get some spring babies!

Depending on your flock and setup the other hens could attack the hatchlings, though. I'd wait until winter's nearly done to let her; then as the hen can no longer fit her babies under and they are a little bare yet the temps will bump up and they won't be quite so cold!