What can I do to help my father who is terminally ill and my mother is not helping?

My father has been diagnosed with a brain tumor which is inoperable. Since this time his condition has been deteriorating rapidly. Here is the problem, my mother will not let any one in the family come visit him. In fact, he's in a rehab center instead of a hospice and she has requested he do physical therapy. My sister and I are wits end because we want our father to be comfortable and well taken care of during his time left. We have spoken to the nurses and they also have noticed how our mother is refusing to listen to the best advice from the doctors and therapists. How do we get our father the care he needs? We believe our mother is mentally unstable and this has only added emotional complexity to the situation. Our father unfortunately goes in and out of lucidness so he is really unable to make decisions even though when the doctor spoke with him on his own he agreed he would go into hospice care. Please HELP!! Our father is only 56 and we can't bare to see him like this! Thank you for your assistance.