I read this nearly 20 years ago in audio and only remember bits and pieces.
In the forward, the author talks about recently breaking his hip.
The book follows two sisters who go to some sort of survivalist camp that is secretly(?) run by aliens(?) or something and learn that the magnetic poles are about to flip.
The big bad happens, and then the book follows the rebuilding. It's sort of, maybe implied that the aliens caused this as some sort of purification(?) or something. Also there are anti gravity cars that work because anti-neutrons are anti-heavy in the novel.