how can you contest a pov

A health care proxy, not a power of attorney, gives a person the right to make medical decisions for someone else, and generally doctors and hospitals communicate with the person who holds the proxy. If you feel that the proxy, or the power of attorney, or your father's will was made under undue influence of your brothers- anything can be contested. You haven't mentioned if you're also being prevented from visiting your father- I'd be able to give a better answer with more information.