Like Steve said, most snakes slow down at this time of year. If you can bump their cage-temps up a bit that would be good to get them started eating more.

Have they ever eaten anything other than worms? If not, you might want to look into getting a calcium supplement. Normally occasionally eating things with bones (fish, mice, ect.) gives them enough calcium, but only worms might be an issue.

Can you get a pic of the tiny one beside one of the normal siblings just to compare the growth?
Try not to worry too much at this point since it is wild-snake brumation time.