Valley of Secrets — Elizabeth Renier

Like a princess held captive in a tower Jocelyn Harmer was a virtual prisoner in her grandfather's ancestral manor, forced to pay the price for the romantic indiscretions of her runaway sister Rosamond. Bethrothed to a man she could not love, Jocelyn dreamed of a time when something, or someone, would free her from bondage.
Then, as if in answer to her secrete prayers, there came a chance encounter with dashing and handsome Captain Adam Peveral. But he bore a name she had been bred to hate, and her heart was forced to rebel against the restraints of an ancient feud and the conventions of 18th Century England.