The Smuggler's Girl by Jennie Felton

In the grand tradition of Catherine Cookson, Josephine Cox and Poldark, comes a page-turning and enthralling new Cornish-set saga of shipwrecks, smugglers, secrets and romance, from Jennie Felton.
She always knew a piece of her heart was missing...
Cecile has been raised to a life of privilege at Polruan House, by her widowed father and aunt. Now she's of age, they are determined that she make a proper match, but Cecile's heart belongs to their coachman, Sam - most definitely not suitable marriage material.
When Sam turns to his friend, smuggler Zach Carver, for help eloping with Cecile, Zach tells of a recent encounter with Lise, a beautiful but poor girl in St Ives, who is the mirror image of Cecile.
And so a daring plan is born to briefly swap the girls. But bringing Cecile and Lise together will uncover an astonishing family secret of a bold escape from a loveless marriage, a treacherous shipwreck and a sister thought lost to the sea long ago...