Is she showing any signs of paralysis anywhere? Drooping wing or twisted neck or limping? Any nasal discharge or diarhea? You could start by giving her some Sav a Chick, you can get that at feed stores and it's not expensive. If she isnt able to stay with her mother and siblings, it would be best to put her in a hospital cage or pen where she can be warm and safe. Normally if it's a situation where it's a sick baby and she is still with mom, I personally would be reluctant to separate her from her family, because obviously it is very stressful for the baby and her mother too, but if she can't keep up then they will eventually reject her anyway. and you will probably have to eventually keep her in a hospital pen so that she can recover. Try giving her the Sav a Chick and see if it helps, but don't waste any time starting her on it, because if they start acting like that, they are very sick.