Only 'encouragement' about our mortality I've discovered in 44 years is that nothing in creation is made to last forever. Stars are born, burn brightly, but eventually die. Planets come and go. And everything alive eventually isn't.
The good news though is everything's made up of atoms including sisters. And when we die, those atoms don't actually go anywhere. They remain. Our bodies stop working, but everything we are remains beyond death. We decompose and atoms go off to become part of other things. When our brains decompose and become part of other things, all our memories and what made us us becomes part of something else. But it's all still extant, didn't blink out of existence because we died. Thus a kind of reincarnation is very real indeed. Whether we continue to experience some part of this new form I don't know - I've never died before.
If you wanna lie to her, tell her that most people don't learn how to please God until they're 80 years old. That she learned in only 10 means God's very pleased with her and looking fowards to seeing her. As lies go, it's not a horrible one.