One Heart to Win by Johanna Lindsey

One Heart to Win - Tiffany Warren's parents, Rose and Frank, promised her in marriage to Hunter Callahan when she was 3 years old.  This was to end the feud between the families.  Until Tiffany turned 18, the Callahans and the Warren's would honor a truce.  Then, without any satisfactory reason, Rose up and left Nashart, Montana and returned to East, taking Tiffany with her.  Now, Tiffany, is reluctantly traveling to Montana, sure that she would hate the west and Hunter.  When she arrives in Nashart, the Callahan's mistake her for the Warren's new housekeeper.  So, Tiffany, masquerades as Jenny, so that she could observe her fiance without him knowing who she was.