Deception by Tarah Scott

Betrothed to Lady Allison, the granddaughter of the dying MacKenzie laird, Jacob MacKinnon stands ready to do his duty and unite their two clans. But enemies watch, and plan to prevent the uniting of those two powers.
After an attack on his betrothed’s carriage leaves only Lady Allison alive, Jacob is determined to find her would-be killers.
Linnae Donald is a lowly serving girl. How is she supposed to tell Laird MacKenzie that the granddaughter he’s mistaken her for died when their carriage was attacked?
She can’t break a dying man’s heart. Neither can she do what Jacob MacKinnon asks and help him prevent a war by pretending to be Lady Alison…and marrying him.