My youngest brother is the sole personal care POA of our Dad (85) who has Dementia & Parkinson's. What are his communication obligations?

He has zero obligation to tell the other siblings anything. He was put in this position by his parent. He essentially is acting for the parent at the parent's instructions, and for the parent when the parent is unable to act, in a manner consistent with what he understands the parent would want. He should follow the instructions of the parent in what is shared and with whom. This is the private concerns and business of the parent, not your business. The best access you have to information is to offer love and support to them both in any way you are able.

As POA it is actually his job to keep dad's health information private. Perhaps dad would sign a HIPPA release for you so that you can find out his health information. I think the reason for this is to reduce the family arguments over the difficult decisions that the POA is required to make. Disagreement and confrontation only makes the decisions more difficult.