Need help!

[happy_feets12]

Hi everyone! I need some advice regarding my hen. She has been acting strangely lately – she mostly just sits still with both legs stretched forward (it looks like she's sitting on her butt).
Her condition:

• Eating/Drinking: She eats and drinks normally (I bring it directly to her).
• Droppings: Normal, no diarrhea.
• Abdomen: Not bloated, feels normal to the touch.
• Supplements: I've been giving her NutriSel Plus(vitamins/electrolytes) in her water for a few days now.

She has no visible injuries, no swollen legs, and no signs of scaly leg mites. Has anyone dealt with this before? Could it be a vitamin deficiency that just needs more time to clear up, or something else (like Marek's or something neurological)?
Thanks for any help or shared experiences!"
Last photo is with her sister yesterday(she is on the left)

 

[happy_feets12]

She can stand up and run,but when other chicken came closer to her she just sits like that. She has also a little bit problems with her balance