More than once I've stood at the grave of someone whose life had been greatly used of God but now had been taken by death -- often at the height of their service. And I have asked myself the same question you asked -- without ever receiving a full answer.
Does this mean God isn't in control, or that He doesn't know what He is doing? No, not at all. God sees the whole picture, while we don't -- and some day in heaven we'll understand. But in the meantime (as Paul wrote), "We live by faith, not by sight" (2 Corinthians 5:7). We live in a sin-scarred world, and at times Satan seems to have his way. But the Bible assures us that someday Christ will triumph over all the forces of sin and death and Hell, and all the evils of this world will be destroyed. Our hope is in Him!
Don't let this person's death shake your faith; after all, her approaching death didn't shake hers. Instead, ask God to help you profit from her example of Christlike character, and grow stronger in your own faith. Her life was a testimony of faith and courage because she had learned to walk daily with Christ. May this become true of you.
As a footnote, I hope you'll pray for her sister. Her good health should remind her of God's love and goodness, and cause her to turn to Christ in gratitude and faith. Pray that she will.