[Ennui.]

It definitely would be awful on your niece to know that she is not wanted, and to constantly be taken care of by someone other than her parents such as a nanny. I'm sorry that she will have to go through that!
Are you able to consult with a lawyer? I can't say this is true in every circumstance since I have never tried, but sometimes lawyers will consult with a person for free and tell the person whether or not they think they have a case that will hold up in court. They will tell you if you must contact DCF (or your state's version) once the emotional neglect starts to happen.
If you do witness anything going on with the child, document it to the fullest extent possible. Write down dates, times, where you were, and what occurred. If you ever get any texts, emails, letters, or photos about your niece that are suspicious, be sure to save them. That will at least provide you evidence for your case.