My sister took my dad to California from Boston and things are getting messy. Any advice?

"She has never taken care of him. I May just let her Have him for a few Months and see if she lasts That Long ."

You just answered you own question. Stop fighting her especially if you want any communication with Dad. She will see how much care Dad is.

Is his apt in a complex or a duplex he or you own? I hope if its owned its own by you. If owned by Dad I see sister, evicting you.

If a complex then you will need to get Dads belongings out of there. May have to pay his rent for Nov if you need time or want to wait to see what happens. Hope there wasn't a lease because Dad may have to pay the balance of it. Your not responsible. If he does pay the balance or even a couple of months for breaking the lease, no one can move in if Dad has paid.

Your sister probably sees that Dad is declining. She wants that inheritance and thinks having POA will get her something. When Dad passes, you can always request an accounting of how his money was spent during her time as POA. Contesting the final accting will hold up distribution of the estate.

My advice would be that any guardianship fight is going to be thousands of dollars, upward of 10,000; siblings fighting over a parent often have the rights to POA and guardianship denied to BOTH entities and given to the state. You can fight and you can attempt to make this a case where winner pays all expenses, believing you will win. There is certainly no certainty of that.
I doubt your Dad was removed kicking and screaming or we would see kidnapping charges placed on day one. At some point he willingly left and willingly signed POA papers. Your argument that he is well enough to decide where he wishes to live means those papers may have standing; as you say, he was not deemed incompetent and you did not have POA. He will be assigned an attorney by the court to represent his own wishes should this case come to court.
Were I you I would start with telling your story to APS in area where Dad lives and ask that he be examined for wellness check and to ascertain his wishes (without Sister present). I would claim "kidnapping" tho do understand, as you did not place that charge on Day ONE it is unlikely to hold much water.
Quite honestly your Dad appears to have left willingly. You can contact Sis with lawyer letter telling her what I just told you, and telling her if she were to win a case she better be meticulous in her care and record keeping as you will pursue her forever UNLESS she meets with you and with Dad in mediation to decide a path forward ASAP.
Without a good deal of time, travel, expense, risk and --as you have already seen a glimmer of-- expense, I do not see an answer to this. As I said, you claim Dad is competent to make his decision to stay alone in his apartment. He left apparently willingly, and if he was not competent to do this should not have been living alone, with your Sister and he chose her as his POA. Rather than make it a war I would consider offering her help, support, and otherwise back completely away, to be frank. There is nothing that bothers me quite so much as warring siblings tearing a senior in pieces when they are least able to fight it. How sad for his end of life.
I hope you find an answer that brings the least pain to Dad. But I doubt that this case will come without GREAT cost, mentally and financially to say nothing of logistically. I am so sorry this has happened.

Here is the link to your Oct 8, 2022 post:

https://www.agingcare.com/questions/my-sister-kidnapped-my-dad-from-boston-to-california-i-will-have-to-file-for-guardianship-has-anyone-477598.htm

"he Is in the mild to Mid stages of Vascular dementia and Alzheimers . We Have a CNA , PT , meals on Wheels, Great PCP and Nuerologist . I doubt my Dad Knew he was going to California and he would be there Permanently ."

"...his Doctor did Not deem His incapacitated . "

Sounds to me like maybe he is incapacitated if he didn't realize what your sister was doing and he needs all the care you mentioned.

We really can't give you advice if we don't know why your sister believed he was not receiving proper care. Unless he actually wasn't. You need to give us both sides of this story. Guardianship is an expensive and time-consuming process. If the judge thinks the family is fighting over someone, she/he may decide to use a 3rd party guardian instead of a family member. Just FYI.

If your father is not incapacitated (as "his doctor" seems to believe), then NO ONE will get guardianship of him, as he will most likely be required to appear in the court to be questioned and assessed. The judge will be able to see and hear for themselves what his condition is. They've seen it all before. You will need to defend what you were doing regarding his care and your sister will need to make the case that he was somehow a danger to himself or being neglected/abused.

If your Dad gave her POA and in charge of medical, she probably had his approval to take him to California. Why did he not give you POA and medical proxy when he was living near you? If he is agreeable to his living arrangement you may not have any recourse.

File for guardianship, too, I guess.

Why does your sister feel he should be with her? If she's expending that kind of money for guardianship, she must be very concerned for his wellbeing. What's her side of the story?