S.C.
My son is 3 and a biter/picker too. I bit my nails from early childhood until I had a toddler of my own to chase around. Since NOTHING my parents tried worked and since I can't think of any behavior he could replace the nail biting with that wouldn't annoy me more I decided to ignore it and NOT beat my head against the wall......
And to get her to stop you'll have to find another behavior for her that meets the same need....