My prayers for comfort and strength have been raised up for you and yours. I remember a friend of mine sharing with me when his wife died how in her last days that she suffered. He was thankful to God for having heard his prayer and providing his wife with healing.
While I know that it is painful, remember the great times with her and be of good cheer, for to be absent from the body is to be present with the LORD. For if your sister knows Christ as her Savior she will rest in his arms and you will see her again.