Defiant Heart (Westward Hearts) by Tracey Bateman

In Book One of the Westward Hearts series, orphans Fannie Caldwell and her two young siblings have spent the last three years as indentured servants under a cruel master. Desperately wanting a better life for her brother and sister, Fannie devises a plan to secretly join a wagon train heading west. Her plan immediately runs into trouble when the handsome yet bullheaded wagon master Blake Tanner refuses to allow an unmarried woman on the train. But Fannie's determined—she'll escape and go west with or without help.

As life on the trail tests everyone's endurance and faith, Fannie soon realizes the perils of being a single woman on the frontier. Witnessing Fannie fending off one scare after another, Blake slowly recognizes how much he cares for this alluring young woman. Will Blake sacrifice his own dreams and guide Fannie to safety? Or will Fannie's stubborn independence keep her from finding true love?