I would suggest you inform her parents or guardians about your discovery first. They need to discuss it with her, express concern, and willingness to offer help. Many children "experiment" but I would leave it to the mental health professionals to provide the care.
Have your relatives talk with their pediatrician or medical home about what they found. In the context of an examination, cutting can be discussed with her as part of anticipatory guidance.
Thanks for your concern about your niece.
Wayne A. Yankus, MD, FAAP
expert panelist: pediatrics