Dad has 2 dpoa's. I'm one my sister is the other. I take him to all doctors spots. She pays bills. Dad has numerous complex health issues with co morbidities. He is very sharp and all will state that. He no longer drives, writes checks, doesn't understand his cash management acvount and how his loan works (collateralized). He is vulnerable and easy to manipulate. Malnutrition and meds play a role as well. My sister yelled at him about selling his house (largest asset) to him for $400k. The FMV is about $750. He usually complies with screamers as he wants it to stop. Is there any recourse? He is 80. Dependent on others. Physically frail. Walking from bed to couch is exhausting. He has left $465 in Roth, condo worth $300k (collateralized) and $200k cash. He said she gets nothing else for inheritance but she did not put anything in writing and still owes him $130k from the transaction. Again, nothing in writing. He doesn't want to deal with anything unpleasant. He feels I say "ugly things" but I'm just trying to explain what has been done.
Sissy is totally taking advantage of a vulnerable adult. Being co-POA doesn't give her the excuse to do this through. Now the hard part in all this is that IF you contact APS on this or speak with good legal on this, that you all could find that the state comes in to establish guardianship over dad. You as coPOA have not done your required duty as POA; and then Sissy has exploited her relationship with dad. Someone with the assets your Dad has is a good candidate for the state to grant an independent guardianship of and a judge would probably do this. Neither you or Sissy would get it. So think carefully before you do things as the result may find Dad completely removed from you or you are limited to time with Dad.
The only way around this - imho - is for you to go to court to get 100% guardianship over Dad. Now you will have to document with bank statements and other financial details all the improprieties that Sissy has done; you will need to do correspondence to Sissy sent certified mail to show you are now trying to set matters straight. You need to probably do this for the better part of next year, and every contact with Dad & Sissy is noted by you with details. Then you go and get your own elder law attorney to go to court to petition for guardianship. You will have to pay for this (I'd set aside 15K for it) but once it's granted to you the $ is paid to you from his assets. None of this is easy and Sissy will likely challenge you on this and the legal costs & court showings could be a lot. Good luck.
What is wrong is for an adult child (or anyone else) to take advantage of a parent's vulnerabilities and cause him to make decisions not in his best interest. If you feel that is going on, and you have evidence, you need to address it through the courts.
I can see how this house sale is not in your best interest -- it reduces the total available for inheritance later. Is it hurting Dad? Is it likely to prevent him from taking care of his own needs?
I'm not trying to say the Sis is an upstanding citizen and I approve of what you describe. But if this is investigated and Dad confirms that he wants to give her a huge break in selling her the house, I'm not sure the law can stop him.
If I were you, I would immediately petition the Surrogate's Court for an independent Guardian to be appointed to save him from the talons of a raptor.