Very unlikely given her age a niacin problem would be developing, however, she may benefit from B vitamins; for that, you can use 1/4 of a B complex tablet, or give a few drops of a Cattle B complex. Musculoskeletal deformities can develop if birds are being fed old or stale food where minerals such as calcium, or phosphorus have depleted. Ensure the feed you're feeding her is not expired, and is formulated for ducks, or all poultry specifically.
Unfortunately without diagnostic tests done at a vet office, determining what her problem is will be difficult. Lameness in the legs such as your bird can be due to spinal/leg trauma, heavy metal poisoning, genetic predisposition, infection, or viral diseases. In addition with the B vitamin nutritional therapy, I would start letting her swim in cool deep water every few days to exercise the legs and reduce any inflammation going on.
Whenever she's not in the water, try to keep the grounds she's on soft to reduce the chance of pressure sore development. If you notice warmness in the feet or swelling, antibiotics such as Baytril may be indicated. Besides that, I'm afraid that's pretty much the extent I can suggest as far as treatment. If she continues acting this way, you will likely find the best outcome seeing a vet in your area familiar with birds.
