I would agree, that since they came on so suddenly. The odds of it being niacin related would be low, especially taking into consideration her age, and the diet she's being fed. That does not nesscarily mean I wouldn't recommend niacin supplementation. It would be a good idea to add a suppmental source of niacin into her diet, just to help with healty leg growth, and structure. You can use vitamin B complex for cattle, and put 1ml over her feed, or give it to her orally. I will provide link below regarding admistering poultry medications by mouth.
I'm not an avian veterinarian, nor do I have any past medical experice, but In my personal opinion, it sounds like there may have been some sort of local tissue trauma to her legs, that has gotten worse over time, and possibly is to the point of (infection).
I'm sure you've done this, but I'd like to readdress the importance of doing so, and that is moving her to a soft-padded area, that reduces her mobility. At this point, we don't want her moving her legs too much, as if it is, injury related, we don't wan't her further worsening it. That's not to say, we want her sitting around like a vegetable all day, we want her to relive some of the weight off her legs a couple of times during the day, and you can achieve that via water therapy. Simply, set her in a deep water area, supervised of course, and let her float around for a few minutes. The cool water, will help a lot with the swelling, but you can boost the effect even more, by adding some magnesium sulfate (Epsom salt) to the water.
If she looks to be eating and drinking well, that's perfect, but what are my concerns if this is some sort of infection, and it goes systemic which will result in some more serious problems. I generally avoid Penicillin when I can, for the reason that several bacteria classes have now become resistant to its properties, an example is Staphylococcus aureus. An antibiotic, I personally think would be better for a case like this, would be Enrofloxacin (Baytril), or Ciprofloxacin. You can purchase both online, (links provided below) and they have a pretty "broad-spectrum" which is what you want.
Usually, when you're dealing with something like this, an antibiotic is prescribed, as well as an NSAID, to reduce some of the swelling and inflammation. Most often, at a vet office, they'll prescribe something like Meloxicam, Carprofen, or Ketoprofen, but if you don't have any of those on hand, there are several products you can obtain without a prescription and use, one being baby aspirin, the other being DMSO gel which can be applied to the deal surfaces of her legs a few times a day. The aspirin, you can dissolve one 325mg tablet in 250ml of water. Just note, the aspirin, is for humans, so there is very little research on the effects it has on birds, long term, so it's best not to use it for an extended period of time.
Among the other things I've just listed, it may be good to start weighing her daily on a gram scale so you check her weight to ensure she's not losing weight. At any point in time, if she's lost 5 to 10% of her weight, you should consider supplemental tube feeding. Here are links to the products, and articles I mentioned.
https://www.backyardchickens.com/ar...dications-to-all-poultry-and-waterfowl.73335/
https://allbirdproducts.com/products/baytril-10
https://www.walmart.com/ip/Fish-Flox-Forte-500mg-30-Count-Ciprofloxacin/124961490
https://www.tractorsupply.com/tsc/product/valhoma-dmso-99-pure-gel-formula-1-lb