The Cornish Bride by Fil Reid

Impressionable and headstrong Ysella Carlyon, younger sister of Kit Carlyon, Viscount Ormonde, makes her long-awaited debut in London Society and is swept off her feet by the dashingly handsome Captain Oliver Featherstone. Who wouldn’t be? But both her mother and brother suspect he is a philandering adventurer in search of an heiress, and they disapprove of the liaison.
Part way through the Season, news comes that Kit’s wife has been taken ill, and Ysella has to return with her mother to Ormonde Abbey in Wiltshire. But she’s determined not to abandon Oliver, and invites him to visit her there. Together, they hatch a plan to thwart her family’s disapproval and elope to Gretna Green. To Ysella, it’s just a big adventure to enjoy, but she’s soon to discover the realities of what she’s committed herself to, and it’s not nearly so much fun as she thought it would be.