Carlos - my experience with hospitals and dying relatives was that while a formal written P.O.A. is useful, ultimately if the family member can't make decisions for themselves the hospital will defer to the wishes of family members on site. However, if family isn't on site, 24/7, the hospital will defer to whatever they think best.
Thing is, the hospital is a massive corporate bureacracy, and shift changes often lead to mis communication - one nurse doesn't know what the last nurse did or didn't do if the last nurse forgot to log everything. Having a family member on site to keep the hospital staff updated is invaluable. Obviously, having a family member on site 24/7 is a serious challenge for most folk.
If business for you is slow and you can I recommend being at the hospital for every moment you are capable in order to advocate for your brother at every opportunity that arises.
"Winston could not quite remember a time his country had not been at war." - Orwell, 1984