Your situation is very hard. As power of attorney, your father has granted her authority to take care of his affairs. If she has a Financial Power of Attorney, she can handle his financial affairs the same way he could. She could even take all of his assets for herself. If she has Medical Power of Attorney, she has the right to make his Medical decisions for him when he can no longer do so. Neither one would prevent him from handling his own affairs while he is able or preventing you from visiting to investigate. She cannot force him to keep her as power of attorney and she cannot force him to live where SHE wants if HE doesn't want to. At any time he can revoke her authority and name someone else.
Until he does that though, she has the power to take care of his affairs. The only way to trump the Powers of Attorney is to go to court to have him declared incompetent and be appointed his Guardian. Each state's laws are a bit different. Generally, you have to be a resident of the state and county in which he resides. A doctor must certify that he is no longer competent. An action is filed in the county Probate Court where he resides, asking for you to become his guardian. The court must evaluate the doctor's evaluation and determine if you are the appropriate person to be guardian. Your sister or father could contest the apppointment, and it can turn into a court case fighting about it. The court will not appoint a guardian if your father is competent and does not need a guardianship.
Instead of a guardianship. you can call the county department of Adult Protective Services and report the potential of financial, physical, emotional or other abuse of your father. They will investigate the situation and assist him if needed.
Unless your father is incompetent and you become his guardian or he removes your sister as power of attorney and appoints you instead, you don't have a lot of rights. I would really try to talk to your father and find out what is really going on so that you can take the appropriate steps. Good Luck!