The most important thing you can do to help your aunt is to turn her attention toward the future God has prepared for her in heaven. This, after all, is our true hope, and it would be tragic to ignore it or pretend it wasn't important.
Right now, your aunt is suffering -- and her suffering is very real and all-consuming. Instead of telling her something you both know isn't true, however, ask God to help you be a good listener, and to express your concern for her. Ask God also to help you point her to the hope we have because of Jesus Christ -- the hope of heaven. By His death on the cross He made our forgiveness possible, and gave us the privilege of being with Him forever. In heaven, "There will be no more death or mourning or crying or pain" (Revelation 21:4).
In addition, let your aunt know you love her and are praying for her. Let her know, too, that you treasure every day you have with her on this earth, and you will miss her when she's gone. Ask if there's anything you can do to help her (such as letting her pastor know her situation).
Someday we all will die and go into eternity. Are you ready for that day? Make sure of your salvation -- as your aunt has -- by putting your faith and trust in Jesus Christ.