Poor little thing!
How old is she?
How is the other one? Do you have photos of her too?
What do you feed including treats?
Were they vaccinated for Marek's?
If she poops, can you get some photos of that too?
The neck turning does look like Wry Neck. I would try giving 400IU Vitamin E and 1/4 tablet B-Complex daily to see if she improves. Egg or tuna would also be good to give as a treat to help with the uptake of Vitamin E.
See that she's staying hydrated, then offer wet feed. Look her over for any injury, lice/mites and make sure her crop is emptying.
Is there any chance she and the other one could have eaten moldy feed, something rotten or toxic?