Point her to Matthew 25:13 "Keep awake for you know not the hour or the day"
Mathew 24:36 But about that day and hour no one knows, neither the angels of heaven, nor the Son,* but only the Father.
John 14:1-3 Do not let your hearts be troubled. Believe* in God, believe also in me. 2In my Fatherís house there are many dwelling-places. If it were not so, would I have told you that I go to prepare a place for you?* 3And if I go and prepare a place for you, I will come again and will take you to myself, so that where I am, there you may be also.
God created us to live faithfully in the world in the here and now and to exercise stewardship over it. The future is in His hands and he doesn't tell us in advance what is going to happen. Tell your sister to knuckle down and get busy doing something useful (environmentalism, social justice, works of compassion ... whatever) and by doing that she will be ready when Jesus decides to shows up.