Can his sister prevent him from seeing his mother at her death bed?

He should demand to see a copy of the POA to see what it covers, as it may not say anything about who can visit. If the sister is not there all the time and she has not instructed the nurses to ban him, he can just go and see her [he probably can just walk past the nurses station without them saying anything]. If that does not work, he should contact the hospital administrator and see if something can be worked out [they can ask his mother what she wants done, as the POA can only be exercised if the mother is unable to make a decision on her own]. Any POA can only be exercised to the extent the person can not physically or mentally do so on their own and can not be contrary to what the person would have wanted to be done. All POAs disappear once the person dies and the person can revoke the power orally at any time.