That's a tough situation to be in for sure. Been there myself. As crazy as it might sound, sometimes I wonder if silence is better (but still praying for healing). For one thing, if the brother who was killed was saved, you can easily say "he is in Heaven now". If not saved, then that's really tough considering the sister you mentioned in a Christian who knows that only those who are saved will be in Heaven.
2 Samuel 12:18-23
18 On the seventh day the child died. David's servants were afraid to tell him that the child was dead, for they thought, "While the child was still living, we spoke to David but he would not listen to us. How can we tell him the child is dead? He may do something desperate."
19 David noticed that his servants were whispering among themselves and he realized the child was dead. "Is the child dead?" he asked.
"Yes," they replied, "he is dead."
20 Then David got up from the ground. After he had washed, put on lotions and changed his clothes, he went into the house of the LORD and worshiped. Then he went to his own house, and at his request they served him food, and he ate.
21 His servants asked him, "Why are you acting this way? While the child was alive, you fasted and wept, but now that the child is dead, you get up and eat!"
22 He answered, "While the child was still alive, I fasted and wept. I thought, 'Who knows? The LORD may be gracious to me and let the child live.' 23 But now that he is dead, why should I fast? Can I bring him back again? I will go to him, but he will not return to me."
John 11:23-26
23Jesus said to her, "Your brother will rise again." 24Martha answered, "I know he will rise again in the resurrection at the last day."
25Jesus said to her, "I am the resurrection and the life. He who believes in me will live, even though he dies; 26and whoever lives and believes in me will never die. Do you believe this?"
Revelation 21:3-4
3And I heard a loud voice from the throne saying, "Now the dwelling of God is with men, and he will live with them. They will be his people, and God himself will be with them and be their God. 4He will wipe every tear from their eyes. There will be no more death or mourning or crying or pain, for the old order of things has passed away."