He has Dementia and Alzheimers. I have always visited and talked to him weekly. All of a sudden she will not let me in the house to see him, What are my rights?

redskin60, as others have said, we need more information to give specific advice. That said, unless your dad has been declared by his doctor as being incapable of making decisions and his POA gives all decision-making authority to your sister, then she does not have the legal right to prevent you from seeing him. Your recourse is to call adult protective services or the police, but before you do that, consider the good questions that DeeAnna and freqflyer have asked and decide whether calling police or ADP will likely benefit your dad or just make things worse for him and your sister. If your dad's condition is such that he requires 24-hour care, your sister's stress and exhaustion levels may have exceeded her capacity to cope.
Helpful Answer (1)
Reply to bicycler

redskin60, can you give us more information.... there has to be a back story here. Yes, your sister who is your Dad's Power of Attorney can limit who and when someone can visit. Something had to have prompted this.

Since your sister is your Dad's caregiver and since she lives with Dad, that means she is doing the work of 3 full-time caregivers each and every day, depending on how far in your Dad's Alzheimer's/Demetria. Is anyone helping your sister? Like paid caregivers or other family members?
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Reply to freqflyer

Who has POA and POA-Health Care for your Dad?
Has something happened between you and your sister? In the past, have your visits upset your Dad either during or afterwards? Do you try to offer "advise" to your sister as to how to take care of your Dad or do you criticize your sister about how she is taking care of your Dad? What do you think might have changed the situation from your sister's point of view?
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Reply to DeeAnna