How do we deal with an adult sister who is isolating elderly father (dementia) and won't allow visitors?

You might try APS Adult Protective Services make a report out-it has to be investigated at the very least.

Thank you very much for your suggestions. We believe he is not being abused, just "used" to hurt my mother-in-law. Other family members, while welcome in the home, are taking a stance of non-involvement, which is incredibly frustrating. While she cannot have the police remove us from the property without a protective order, she does not have to open the door if we show up. We would like to know if there is a way to force her to allow us to see him, as I really don't think she will do it voluntarily. I will look for the advocacy group in his state and hope for the best. Thank you all again for your responses. This situation reminds me how blessed I am in my own family, who are all very close and would never dream of behaving like my mother in law's family. Thank goodness for good people!

Contact the Adult Protection & Advocacy organization in the state where the father resides. If the father does not have a power of attorney then you or another family member should get the POA papers typed up, signed & notarized. The best solution would be for someone to petition for legal guardianship. The POA can be done without a court hearing, but the guardianship has to go through the court. It's possible that the sister is exploiting him financially & that is considered elder abuse. Best of luck to you & your family!

To check on your legal recourse, call the department on aging of the state where the gentleman with dementia resides. Explain the situation and ask for steps that the daughter who is estranged from the sister (but not from her father?) can take.

Estrangements within families are so sad! I suppose some are justified (incest, abuse, etc.) but they always cause pain to innocent parties.

I wish you the best in trying to intervene.

What about a friend of the family that can check on him? Does she have anyone that she respects and likes enough to let them in? The whole thing sounds fishy to me. I'd be worried too if I were you.