My brother is the DPOA of my Dad (91) with dementia. He is withholding any medical information from me and my sister. What do we do?

Unfortunately, nothing. Due to the HIPAA law, the clinics and doctors cannot release any medical information without some kind of document. Therefore, you're at the mercy of your brother. Are you able to put aside any sibling resentments, hurts, etc... and try to make peace with brother? Just keep reminding yourself that this is Only Temporary. No matter how much bro pisses you off - 'it's only temporary. Does he have FaceBook and do you know if he or his family posts anything about your father?