Silver Thread (Chronicles of the Golden Frontier #4) by Gilbert Morris

A sister missing. A brother searching. Each needing an answer to save a life.
Abby DeSpain is missing--gone without a clue! But her brother Grant has his suspicions. The very idea that Abby could be chasing after that rogue Billy Rogers and running straight into trouble spurs Grant onto her trail. Boomtown after boomtown brings him no closer to finding his sister, but in each he encounters characters who intrigue him and challenges that build his character--all of which guide him to astonishing answers to his life-long questions.
Abby is indeed searching for Billy. He's left her a trail of letters promising her the world. Promises she feels she deserves--and ones she intends to make him keep. But when the trail abruptly ends in the most lawless boomtown of them all, trouble finds Abby quickly, and she soon learns that her normal tricks do not work.
With time running out, Abby realizes she must ask for help from the only one who can save her--the one she abandoned long ago.