I feel for you in this most heart wrenching time. It is horrible for anyone to witness a loved one go through such agony. I wish you much strength, and pray that you do not lose hope.
There is no way to avoid the pain that comes with loss. But there is an approach to it that can make the pain slightly more bearable. This comes by learning to focus on the soul rather than the body.
The body is limited and finite. It is born and it dies. It can be healthy and it can get sick. No one is here forever. No body escapes death.
But the soul lives on. The soul has an existence that is beyond the confines of life and death in this world. Our time here is merely a short chapter in a long story. During our lifetime, this world is all we see. But deep down we are aware that there is more. We know that while our material possessions and worldly achievements will die with us, our spiritual wealth and the good we do will live on. The soul does not die.
Your brother's body is weak. But his soul is strong. His true self, his inner self, is very much alive and will continue to be so. And it is his true self that you love, that you are connected to and that you yearn for. The brother you love is the person he is, his character and his presence, not his flesh and bones. Ultimately it is not his body that you love, but his soul.
The more soulful you are in your own life, the more you will be able to stay connected to your brother's soul. Through prayer, through Torah study, through charity and good deeds, you become more attuned to your soul and your spiritual side. Become more in touch with the real you, and you will see beyond the suffering body to the real him.
Of course, we would rather just be able to connect with our loved ones like we always did, to talk and laugh with them, to hear and hold them. We don't want our relationship to have to become abstract and spiritual. But sadly, this is the reality, until G-d puts an end to all suffering and death, when Moshiach comes. May it be soon. May it be now.