I've had a number of friends over the years who have experienced what you're facing, and I understand at least something of the heartache and sorrow this terrible disease causes. My heart goes out to you–but more important, God knows what you're going through and His heart is touched by your suffering.
When something bad happen to us or to those we love, it's natural for us to ask why God has let it happen. To be honest, I don't have a full answer to that question, nor does anyone this side of heaven. We live in a fallen, imperfect world, and life is riddled with sorrow and pain. As the Bible says about our years on earth, "their span is but trouble and sorrow" (Psalm 90:10).
The real issue, you see, isn't why bad things happen, but how we should react to them. Will we react in anger and bitterness or will we respond in faith and trust? Anger is a dead-end road–it only hurts us and those around us, and doesn't solve anything. But faith gives us hope–hope for the present and hope for the future.
You face a very difficult road during these days but I pray you will not travel it alone. Instead, turn to Christ and thank Him for His presence and strength. Jesus' promise is true: "Surely I am with you always, to the very end of the age" (Matthew 28:20).