Guardianship/Caregiver Rights

My niece has been under the guardianship of my parents for the last few years. It is not a legal guardianship. My sister is not inteh picture and she nor the father support my parents in anyway for the care of my niece.
My father has taken ill and my mother would like to send my niece to me. We know my sister wont like it but until this point she has not been any type of caregiver to my niece. My mother just wants to send my niece down here to live without consulting with my sister due to fear that if we do discuss this with her she will become irrational and threaten to take my niece (who she cant care for).
What are our options? My mother can prove that she has cared for my niece with receipts for food, clothing, tax returns, and schooling for the last 5 years. She has the birth certificate, social security card and all medical information. If she does send her to me, with all the documentation, will i be able to get her into school?
What can we do? - Is this your question? Add additional information