A while ago I read A Heart Divided by Kathleen Morgan. Here is a book description:
"It is 1878 and the Caldwells and Wainwrights have been feuding for decades. Still, Sarah Caldwell has misgivings when her father pressures her into distracting a ranch hand while he and her brothers rob the Wainwright place. When it becomes clear that hand is actually Cord Wainwright, Sarah realizes she needs to lay low. But Cord spots her in town and, with the sheriff away, makes a citizen's arrest, dragging her off to the Wainwright ranch until the sheriff's return. As the feud boils over, Cord and Sarah make a most inconvenient discovery--they are falling in love. Can they betray their families for love? Or will their families betray them?
Against the beautiful and wild backdrop of the Rocky Mountains comes this sweeping saga of romance, betrayal, and forgiveness from beloved author Kathleen Morgan."