We have POA and are in process of declaring them incompetent (doctor's letters etc. to the lawyer) This is going to really make our dad very upset - he has dementia but is very aware. He is paranoid - doesn't trust us, etc. How do we gain access to their financial paperwork if he gets angry and won't give us checkbooks, bills, etc.
You have several options since you have POA. When you go to the bank just find out what your options are and which is most suited to your situation.
If the parent is known to be unreasonable, overly controlling, rigid, and oppositional - then the children are in the very uncomfortable postition of having to take charge without the parent's consent. It's necessary that they do this, if they've accepted the role of attorney-in-fact.
This isn't intended to be critical, but I read your post several times and couldn't help get the impression that this is somewhat of a power grab, that you and your sister are determined to take this action one way or the other.
You don't indicate if there are factual justifications, or what your mother's status is.
Have you tried a "Mom and Dad, we'd like to help you approach first?"
That approach may not work, but your parents are entitled to know what you're doing. They apparently trusted you enough to name you as their attorney-in-fact, so you need to show that you're worthy of it.
And unless there are facts that haven't been stated, I really do think that going their documentation while they're not home is very offensive and invasive.
Try to think of a less confrontational method of handling this and show that you are capable of handling their affairs on a professional basis.
What I'd probably do (if dad really IS incompetent) is to get them out of the house and then find all the paperwork myself, confiscate it and take over the responsibilities. I'd let him know what I was doing after the fact and just accept that he was going to be angry.
I'd discuss every single check I wrote with him and give him a monthly statement of his accounts as long as he was interested. After a while, as his dementia progresses, he won't care anymore.
I'd only do this if I REALLY BELIEVED he was incompetent, though. It's the nuclear option.
Also, it's helpful to get copies of all his credit cards and i.d. -- whatever he carries in his wallet. Some institutions ask for a copy of a photo i.d. And having all of his credit card info is very helpful (get the security code on the back too).
If he drives, get his car registration info as well. And copy down all the info on his prescriptions as well. Begin communicating with his doctors if you haven't done this yet.
That ought to make a dent in things :) Last, now is the perfect time for you and your sister to get comfortable with taking over things. It feels very disrespectful and sneaky in the beginning, but it's necessary, and the sooner you get used to being in charge of these things, the better. Good luck!
I would assure Dad that you will keep him informed of all financial dealings and go over bank account each month. It is still his money and perhaps this will make him more comfortable.