The Blackstone Twins, Coup de Grâce: The Blackstone Twins Saga, Book Three

Late in the 22nd century, twins Zoe and Zachary Blackstone are born into a world of nightly madness. Having lost their mother, they are dependent on their grandmother to guide them and a misfit band of wishful dreamers, as they journey hundreds of miles, in hope of finding a cure for the Night Madness, the mysterious affliction that takes all men. Zachary is doomed to share this fate, but for a rumor, a possible cure. Struggling against the odds, and up against dangerous wild animals, wild men, treacherous terrain, and a psychopathic woman, his twin sister Zoe is determined to help him discover this one hope of relief, so they can possibly return to a normal life, a thing of which they've only read about in books. The Society that's developed is a strong matriarchal society, governed by mothers.