Keeper of the Dream — Ann Simko

The plan is simple: Save the world, erase your own existence.

Thrown thirty-five years into the past, Derry and Taylor Brandon have the chance to alter the events that ended humanity. Easy, right? All they have to do is track down a father that hasn't conceived them yet and convince him of who they are and that he has a hand in destroying the world. Then they'll be able to fight the man who has tormented them in their own time--and now in the past--and confront and conquer their personal demons along the way.

Piece of cake.