She is having myoclonus seizures, caused by stress. It is localized to her head and neck with the jerks and twitches at this time, but she is sitting still. Have you witnessed her having an episode while standing or walking? She'll likely roll over backwards and either continue to roll or go to her side, thrashing. I would not breed her, or stress her in any undue way. This may pass in time, it may increase involving more of her body functions. It's really hard to tell what causes seizures other than it is some kind of disturbance in brain activity. I would wait out breeding her for several months to see if she passes out of this stage and becomes seizure free. Sorry for you to experience this. She should be able to lead a relatively normal life if you keep her quiet, happy and free from stressors. Brooding chicks if she is still in the same condition as she is now would be a disaster to her and her system, as the mother hens are always on alert.