The heir - Johanna Lindsey

The grandson of a Scottish clan chieftain, Duncan MacTavish, is shocked to learn that his other grandfather was an English marquess whose title and London estate he has inherited.
As if that weren't enough to deal with, he's engaged to a stranger who has publicly mocked him.
His bride to be, Ophelia, is more concerned with guiding a lovely and inexperienced country girl named Sabrina through the latter's all-important first coming-out season than with her own wedding.
The enchanting Sabrina enthralls Duncan much more than his intended does, and she likewise shares his passionate feelings.
But duty and circumstance - plus a scandal lurking in Sabrina's family past - have made their union an impossible dream, unless true love can somehow, miraculously, find a way.