My husband's sibling has POA of their mother but will not share the document. We do not know if it is a medical, financial or any other type. My MIL has dementia and has no idea what she signed or when, She lived with us for 5 years before needed to go to long term care so the POA came as a total surprise.
I would say go see an attorney maybe he or she can get a copy of the POA paperwork. Guardianship might be the way to go but its expensive. You can also do what my family did to me turn them into Dept of human services or social security to make them prove what they have in paperwork if they are spending the funds right.
Prayers
Consult an attorney anyway to see if there's any was to write a strongly-worded letter about sharing the legal paperwork. They aren't necessarily obligated to do that, but she may not know that.
Once u know for sure the person holds POA then they are in charge and do not have to make you aware of financials or Medical. They become Moms rep and carries out her wishes. Should not be using her money for personal reasons unless specified in the POA.
As MIL is in care you can rest assured that someone is taking care of the financials. Ask her facility to reassure you that her financials are being taken care of by a POA and that they have papers for her POA medical and financial.
I would ask BIL to simple assure you that he currently has medical and financial POA over your MIL. You can also try a lawyer letter requesting reassurance that MIL has adequate support so that you do not have to apply for guardianship (it is too late for any POA changes by her).
If you suspect any FRAUDULENT activity, or have reason to believe it is happening with finances or sales of assets and etc. it is time to see an attorney.
But, again, a POA need not disclose nor discuss documents to do their duties by their appointing principal with anyone whatsoever. And I doubt you will get more than a letter of assurance, or some comfort in the facility MIL is in reassuring you that there is a financial and medical POA. You certainly won't get a copy of the documents if you are estranged from the brother.