The other ladies have given such great advice!! They are right, you must first contact a lawyer.
I just wanted to add a little more. Remember, YOU have always been there for this little girl, and you will always be there. This woman, ANN, will not. For some reason, whether it be her parents or what, she has decided to give mothering a try. This will NOT last, and you need to be that constant, warm place to fall when ANN leaves again for some other man. She will NEVER have the bond with ANN that she has with you and will NEVER be able to take that away!! As far as the grandparents go, they raised this woman, so they can't be "all that" themselves. NEVER doubt your place in this little girl's life and heart. You will not lose this little girl, not in custody or her heart!!