Highlander in love / Julia London.

Enhanced descriptions from Syndetics:

The Lockhart brothers' quest for an ancient family heirloom has proved fruitless...and an unpaid loan has left them indebted to their rival, Payton Douglas. With no recourse, the Lockharts offer Payton their most valuable commodity: their sister Mared's hand in marriage.
Raised to despise the Douglas name, Mared outwardly agrees to the marital bargain for the sake of her family; secretly, she concocts a plan to ensure Payton will not wish to marry her. Seeing the handsome, virile laird Payton face-to-face, however, threatens a centuries-long enmity -- and awakens Mared's deepest desires. But she will not be swayed. Choosing to repay her family's debt by working as his housekeeper instead of wedding him, she finds herself inexorably drawn to the man she vowed to resist. Then a profound discovery changes everything....Now, no longer bound by a promise to her family, Mared must let her heart decide if the laird Payton is the enemy of past legend -- or if he holds the key to a future filled with a passion beyond her wildest dreams.