The B complex usually has C added to it, but check your label. I hope that she gets better in a few days. Keep her on mostly chicken feed except for the egg. You can moisten it which mine always loved.
I know that selenium helps in uptake of vitamin E, but have not read about giving it with B2 (riboflavin.) Scrambled egg (a few bits) has plenty of selenium in it. Sunflower seeds (a few) as well.
Prompt treatment may reverse the symptoms. If improvement is not seen in a couple of weeks with B complex containing riboflavin, then I would worry about Mareks disease brought in by new additions, or some other issue. Here is some reading about riboflavin (B2) deficiency...
I agree that human B complex or Super B complex is good for treating a possible riboflavin deficiency (sitting and walking on hocks is a symptom.) Give 1/4-1/2 tablet daily, and you can crush it in food or give it for her to swallow whole. Is she curling her toes under?