My brother is the POA for my mother for both Health and Financial Care. Is he obligated to share who the witnesses are to the document?

Your brother does not have to show you nor does he have to share any information with you. I know this is probably not what you want to hear, but your mother has the right to her privacy and as a POA it is your brother's job to keep that privacy!

I think you may have a right to see the POAs documents, but no rights to bank statements or medical records! You can call a lawyer and ask him/her if you are entitled to see the POAs documents. Most lawyers will answer your question for free or for a low fee!

He has no obligations to the siblings.

You could check with your mother's bank to make sure they have a POA on file for her account. They probably won't show it to you, but they might confirm as to whether one actually exists.

Unless the rules vary from state to state, I don't know that witnesses are required to make a POA. My parents' POAs were notarized, but there were no witnesses to them. (They were prepared by an attorney, however.)

Is your mom competent enough to confirm that she made his POA and to encourage him to be more transparent?

He cannot be pressed in any way by anybody.
He need not share any information with anyone, just as in the same way your mother need not share her OWN information if she is competent.
He has been put in place by your mother and at her request to act as she asks him to act for her while she is competent, and then if she is incompetent to act as best he is able to act FOR her as she would have wished him to act to the best of his ability.
You can certainly ask your brother to share with any "second" named that they ARE named, so that they can step forward in the event the brother meets an untimely demise or incapacitation. Other than that you can also offer him your assistance in any way you are able to assist, and ask that he be so kind as to share with one of you any important health information he feels able to share so that a "phone tree" can be formed to inform concerned and loving family.
An adversarial relationship with your brother will net you nothing. A cooperative one may or may not.
If, on the other hand, you and your other siblings believe with some evidence that the brother is acting fraudulently, claiming a power he does not have, you can together visit an Elder Law Attorney to ask how to proceed, or open a case with APS presenting to them your evidence and suspicions.