If your sister is competent she can make the decision to change her POA and to leave the nursing home. Niece would not be able to stop her mom's decisions.
Lorraine, has your sister's daughter been her caregiver at home? If so, there does come a point where either the patient needs a higher level of care, or the caregiver becomes totally burnt out and overwhelmed.
Could you give us more information about your sister, where she was staying prior to going into a nursing home, and what are her medical issues. Were you ever her caregiver?
And if you are able to move your Sister, where would you want her to be? Would you be able to be her full-time caregiver?
Your profile says the person you are caring for has mobility problems. Is that still true? Have you planned how you would be able to provide all the care in your home? I'd explore why she's there, to confirm getting her out from NH is feasible. Do you know why her POA wants to keep her there? More details might help others respond to your question.
Could you give us more information about your sister, where she was staying prior to going into a nursing home, and what are her medical issues. Were you ever her caregiver?
And if you are able to move your Sister, where would you want her to be? Would you be able to be her full-time caregiver?