This sounds like what I have, and I've had it for quite some time, but have just recently found out what it is; it's called Dermatillomania (look it up, research it, you'll find it)- Obsessive compulsive self injurious skin picking. It is almost like cutting, but a slightly different form of release. I pick at myself to the point of injury, especially at my face, back, head and fingers (cuticles). I now have acrylic nails on to keep from causing AS MUCH injury (I know she is too young for this), but now I will sometimes use "things" to pick with, like cuticle scissors, or nail clippers, etc. So, yes, this is something that she needs help with. Unfortunately, I can't really give you any recommendations as to what works, but I think taking her to her pediatrician and/or a specialized children's therapist is a good idea. Don't blow it off, otherwise she could be like me, an adult going through the problem. I do know also that it is associated with anxiety/stress and is a part of OCD. That doesn't mean she HAS OCD, but it is a counterpart, something that is related to OCD.
This behavior doesn't necessarily mean that she is having "issues" right NOW, especially if this is a behavior that has been going on for a long time. It could just be her, part of who she is. But, if it's a newer behavior, then it could certainly suggest that there are things that need to be figured out, discussed and dealt with. How is your relationship with her? By your reference that she is a "bonus" child, to me, that suggests that it's not so great, and you don't consider her yours, or part of your family....? Just my perception.
Good luck, I hope you guys are able to help her.