Having been prepared, your sister will have started the greiving process even while her husband was dying, but there is still that sense of numbness when it happens, a kind of shock when you go through the motions of practical life, the blow finally fallen. When you lose a loved one that close you never get over it. You will miss them the rest of your life, but the first years are hard. She will need you to be available and to listen and comfort, even if it is a phone call at midnight.