Sick hen, tail and rear end tucked under with low energy

[mominator60]

I have had 2 of my 4 hens die. A 3rd I believe is dying & it's really too late to separate her from her sister. I feel like a failure because I have no clue what's wrong & can't afford a vet. They walk like a penguin & are in obvious discomfort. The poop is black with white. The first one just became listless & was sitting by the pen door all night. In the morning she was dead. The second one started the same way so my son tried to see what was wrong. He told me there were maggots eating her alive so he & dad put her down. Now like I said, the last 2 seemed doomed unless someone can tell me what this is & what to do. I feed them crumble with oyster calcium added by myself, dried worms & spring mix.