Good morning Becky,
I'm Doug, and I'm very sorry to hear of your situation. My goal is to provide you with excellent service today. I am a licensed GA attorney.
Unfortunately, unless the parent(s) of your niece are deemed unfit parents by a GA court, there is no way for you to legally acquire legal guardianship over your niece without the express approval of your niece's parents.
Legal guardianship would have to be granted to you by a GA court at this point in time as that is where your niece's legal residence is, and seeking legal guardianship will require the consent of the parents and approval by the court.
If the mother has changed her mind and will no longer agree to allow you to take custody, then I'm very sorry, but there simply is nothing that you can do.
I understand that you may be disappointed by the Answer you received, as it was not particularly favorable to your situation. Had I been able to provide an Answer which might have given you a successful legal outcome, it would have been my pleasure to do so.
If you have additional questions, you may of course reply back to me and I will be happy to continue to assist you further until your questions have been answered to your satisfaction.
Would you please take a moment to positively rate my service to you based on the understanding of the law I provided by clicking on the rating stars---three stars or more. It is that easy. That is the way I am compensated for having helped you.
Thank you in advance. I wish you the best in your future,