Fight or Die by Christopher Kenworthy

He's carrying Army dispatches and trying to avoid the Indians.  The Apaches didn't like anybody and he didn't want to one of their victims.  When he stops in town for food and some rest, he meets some of the members of a wagon train.  They are planning to travel the same route he did.  He warns them about the Indians and asks them to wait for his return.  Then he will guide them on the trail and help them avoid the Indians.

Pioneering Press let me read a copy of this book for review (thank you).  It has been published and you can buy a copy now.

When he returns, he finds out they didn't wait for him, they left two days.  He's afraid of what he'll find and he's right.  The mother and father and two children are dead.  The oldest boy is badly injured and the older sister is gone.  He gets the boy healthy enough to ride and then they go after the sister.  He's afraid of what might be happening to her but he's not mentioning that to her brother.  The boy has enough other devils to fight after watching his parents die.

With crazed soldiers, wild Indians, and the cat and mouse games Indians play, this whole quest to get the girl back is dangerous and life threatening the whole way.  Having a new in command who only knows how to polish his sword means disaster.  He's always commanding the soldiers to do something stupid so they quit listening to him.  He also keeps trying capture the rescuing team.

They finally survive this ordeal and he begins to worry about what will happen to the boy and the young woman.  When the doctor is through checking them out, he tells him the young woman already has a plan...