I understand your hurt, but I want to assure you that God understands your hurt even more than you do and wants to surround you with His love and compassion during this difficult time. The Bible says that He truly is "the Father of compassion and the God of all comfort, who comforts us in all our troubles" (2 Corinthians 1:3-4).
I don't fully know why God restores some people to health but doesn't do it for others. Some day in heaven we'll understand, but in the meantime we can only look to Him in faith and trust. If we don't, we'll only end up in bitterness, anger and confusion -- and that's a dead-end road. Remember: God knows what it is to lose someone to death. His only Son died on the cross for us.
But let me ask you a question: Where is your little niece now? Death has taken her, and you can't help but grieve over her passing. But listen: She is in heaven now -- and all her pain and weakness are gone. She has been healed -- completely and forever! The Bible says that in heaven, "There will be no more death or mourning or crying or pain, for the old order of things has passed away" (Revelation 21:4).
Never forget: Because of Christ we have hope. In the midst of your tears, ask God to help you lift your eyes to heaven and the promise we have of eternal life through Christ. Is your faith in Him?