Machina, I am so sad to read of this situation. What a dreadful, dreadful end.
I think it is very important to understand that your brother-in-law's medical issues didn't just appear. He has likely lived a lifetime of medical issues, and there is nothing that can turn back the hands of time or undo the physical damages that he has experienced. The best thing that everyone involved can do, today, is to look to this man's comfort and spiritual state of mind. After so much medical damage, I can only imagine how his spiritual necessities have suffered. And, this is the time to tend to those needs, today.
While I would like to support any miraculous event, supplement, nutritional direction, or protocol, it would be indulging in false hope. So, rather than trying to prolong his life, it might be a wise option to help him to live in the moment - in the present.
I am so, so sorry for your brother-in-law's conditions and for the helplessness and hopelessness that everyone is feeling. Please, try to keep in mind that "hope" for healing can include spiritual/emotional health in an end-stage transition.
May you and your family be comforted by blessings of courage, compassion, and strength during this most dreadful time.