What your brother is doing to himself is his decision. Pelican is entirely correct.
For your own sake, do not take on guilt or remorse for his condition.
What you can do is talk to your brother and straight out ask him if he will give you or a sibling medical power of attorney, and what his wishes are if he becomes unresponsive. This is the best kind of love you can show- that you respect his wishes for his own medical care and that you are not hiding from his obvious horrible condition.
I was with my own brother in the final days of his battle with cancer. The last few days he was unresponsive and his wife and I took care of all his needs. As I drove home I prayed that God take his life soon, because his suffering was so great and his body was so thoroughly destroyed. The next day he died peacefully. And I thanked God for that.