How do I petition my father's Power of Attorney for his financial information?

The problem now is Dad has Dementia so really cannot give you permission. If he could, he would need to write a letter to his POA saying it was OK for them to give u his financial info. There is no formal request. As said, POA answers to your Dad. You can call the POA telling them what Dad said or write a letter. Just don't be surprised if the answer is no.

You do not need to respond but what do his step-children have to do with this. As steps, they are not entitled to anything legally unless he adopted them. Did he raise them? Are they afraid that Dads care will use up all the marital assets? If so, his wife would need to see an Elder lawyer to have their assets split. Dads split eventually going towards his care. If step siblings have a problem, they should be talking to the POA and not you. They are entitled to protect their Moms assets too. The POA also should be made aware that there is a problem because that person handles Dads finances and should be protecting them. You should not get involved because nothing u can do.

Really?
I didn't know a POA COULD share financial information with family just because they request it.
In normal circumstances they cannot.
Perhaps your Dad, when well enough, or now still BEING well enough, has OK'd this release of information.
Generally, a POA acts for the person who requested they serve as POA, acting with the person or for the person if the person cannot act him or herself, but that POA is under no obligation to share any information with anyone but a COURT if the court requests an accounting (say a family suspects fraud for some reason).
I wish you luck.

Your Dad's legal PoA has absolutely no legal, moral or ethical obligation to disclose any of your Dad's private, personal info to you or any other family member or person, whether you write a nice letter or not. Even if another attorney wrote a letter to the PoA on your behalf, you'd need a legitimate legal reason (like suspicion of financial abuse) for a judge to force disclosure.

What is the "difficulty" you are having with your step-sibling that requires knowing your Dad's financial information? Why doesn't the step-sibling ask the PoA themselves? Is this an inheritance issue? Hopefully not. And if it is, this is the domain of the Executor of his estate, not the PoA. And not until your Dad passes.

It seems to me if this Agent is so wonderful and accommodating they can tell you exactly what they require in the way of a formal request.