I agree with Rebascora in that it looks like possible Mareks. Hopefully not. She is very experienced with Mareks, and has good info in her threads. It's usual to get a new chick with Mareks, when your older girls have not been affected. Usually, the disease is in your environment. Here is a long article to read from another BYC member, kind of a "term paper" on Mareks:
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/