One heart to win / Johanna Lindsey.

Enhanced descriptions from Syndetics:

#1 New York Times bestselling author Johanna Lindsey presents a passionate novel of hidden identities, family secrets, and a love that transcends a fifty-year feud, set in the majestic ranchland of 1880s Montana Territory.

Some young ladies marry for money and social standing, a few lucky ones marry for love, but Tiffany Warren is marrying to end a feud. Honoring her mother's wishes, Tiffany reluctantly travels west to meet her estranged father and his enemy's eldest son, rancher Hunter Callahan. Once the Warrens and the Callahans are united by marriage, both clans will stop squabbling over a disputed strip of land. But in the chaos of a train robbery Tiffany seizes a golden opportunity: By assuming the identity of her father's new housekeeper she can live with the father she never knew and assess his true character, as well as that of the neighboring cowboy to whom she is betrothed. But, too late, Tiffany discovers that the rivalry between the Warrens and the Callahans has escalated when the Callahans steal the Warrens' "housekeeper" as soon as she steps off the train!

Now Tiffany, masquerading as Jennifer Fleming, finds herself living in the enemy camp, under the same roof as her fiancé. All too soon she learns her intended is a handsome, sweet-talking charmer whom she has to fight off because he can't keep his eyes-or his hands-off Jennifer. After Tiffany's charade is exposed, she refuses to marry Hunter, not even to end the hostilities between the families. How can she wed a man who is in love with another woman? As Hunter goes about claiming his rightful bride-to-be, he knows that although he loves two women- proper, elegant Tiffany as well as spunky, passionate Jennifer-he has only one heart to win.