The Last Cowboys of San Geronimo by Ian Stansel

This is the tale of two battling brothers.  They both work with horses, Frank is married but Silas isn't, and they couldn't be more different from each other if they were strangers.  They play mean tricks on each other, fight when they have too much to drink, and Frank even shot Silas once.  Now they aren't speaking to each other anymore, so why is Frank on his porch?

Houghton Mifflin Harcourt and Net Galley allowed me to read this book for review (thank you).  It will be published July 4th.

Lena loves Frank and has stuck with him through good times and bad.  She took care of his father while he was dying of cancer.  When his father didn't recognize him, it shattered his world.  Now he finds out he's got the same disease.  He asks his brother for an impossible thing: To kill him before he gets bad.  Silas doesn't want to, but his brother tells him if he ever loved him to pull the trigger.  Silas does.  Then he gets on his horse and rides away.

Lena climbs on her own horse and heads off on his trail.  This is set in present day but there are plenty of woods and greenery on the coast and the horses take them places cars can't go.

As they traverse the countryside, you get the backstory of the cowboys and by the time Lena catches up to take her revenge, she realizes what must have happened.  She doesn't kill Silas.  

The ending is sad but not unforeseen.  Life was never easy for the Van Loy family.