I am sorry to hear that your sister is in a difficult situation. In this matter, ARS 25-409 controls, which I have linked at the end of this post. There are certain factors that must be present for a third party, someone who is not a legal parent, to petition for legal decision making and physical custody over a child. Please See ARS 25-409(A) and (B) to view the requirements.
My recommendation would be that you obtain a free consultation from a family law attorney to review your case before taking action.
Judd S. Nemiro
Law Offices of Judd S. Nemiro, PLLC
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