For the most part, the answer is "no." The agent under a financial durable power of attorney has no obligation to disclose financial matters to anyone other than the person for whom he is acting. But the fact that your brother may be refusing to pay for care that you are authorizing as your father’s agent under his health care POA gets into a gray area.
You need to be able to work together and you cannot make appropriate decisions for your father without knowing what he can afford. Your brother may argue that he can just tell you what’s affordable and what’s not, but it can be very difficult for you to do your job without cooperation and more transparency from your brother. He should be more transparent. Ultimately, he’ll have to be since after your father’s death you will have the right as an heir to get access to his financial information.
Get information on resolving disputes between medical and financial POAs in this article.