There is lots of evidence that this behavior and other related ones are hereditary. It's always hard to see our sweet children become afflicted with this, but I think it really is more an affliction than just a dirty habit.
My 3.5 grandson has started biting his nails, mostly when he's going to sleep. I'm a life-long nail biter (although mostly abstaining for decades). My daughter tears at her cuticles. My sister digs at her skin. My mother was always picking at all of us, both verbally to get us to stop our picking and chewing habits, AND, ironically, to physically remove little bumps and scabs from our tender skins. (She was afflicted, too.)
I notice that I'm still most susceptible when I'm bored, nervous, or haven't had enough physical activity. Sitting in meetings or watching movies are my weakest times, so I always try to have something to manipulate in my hands to help avoid starting that nervous activity. When it's happening, I'm not always aware of it until the "need" to keep chewing has kicked in, and then, as you probably know, it's much harder to stop.
I also tend to start working at tiny rough spots in the edges of my nails. If you can keep your little girl's nails trimmed and filed smooth, she'll be less likely to start nibbling at them. And THANK YOU for keeping toxic nail polish away from Mira.
Unless she reaches a point where she really wants to stop, you are not likely to be able to "make" her. In my experience, your trying to do so may just make it worse, and she could do it more compulsively or try to be sneakier about it.
This request was made by another mom just a week or so back. You might find other suggestions you like here:
http://www.mamasource.com/request/5321340175891103745