It certainly does not look like yolk sac...i have seen similar pictures of intestines, but the chicks were dead, and I can't tell if those chicks are alive or not. I know the last post is from a week ago, did the chicks make it? If it dried up and fell off, then it was yolk sac. And no, there is not a whole lot you can do, other than keep them warm and hope for the best.