The book is "The Last Ride" by one of my favourite contemporary western authors, Thomas Eidson (although it has been reprinted since the release of the film under the title "The Missing."
It constitutes one part of a thematic trilogy which features the strengths of women. "Agnes' Last Stand" centred on Faith, "All God's Children" on Friendship and this one on Family. In all these books strong minded women are featured who act as the lynch pin in the face of harsh realities.
I'm not sure Eidson had the ideas Scherps outlines in mind in his book but I'm a firm believer that the meaning of all art is that which the reader sees in it. We all find different parallels and resonances which make interacting with a particular work (be it a book, a film, a painting or whatever) relevant to our own lives and experiences. Whether the original artist had these in mind doesn't matter too much in my opinion. The death of the author and all that. If it makes sense to you to see themes in one way and you can explain why, that's good enough for me. That's what art is all about. We all have our own interpretations and the work will speak to each of us in a different way.
For my money, it's a story about the conflicting natures of humanity and those innate ones that finally hold us together. But that's just me.