A Castle in the Air by Kelley Armstrong

Portia Hastings has spent her life rescuing her younger sister from one scrape or another. Now, she has to step through time to do it, following Miranda from the nineteenth century to the eighteenth. Almost immediately, Portia is beset by highwaymen, which might be the most exciting thing to happen to her in a long time, even if she'd never admit it.

When one of the highwaymen is injured, Portia makes the inexcusable mistake of helping him-she is trained as a doctor after all. Her travel companions abandon her, and she's left at the mercy of a highwayman who demands she continue tending to his compatriot's wounds.

Portia soon finds herself at a crumbling castle inhabited by the destitute Earl of Ravensford. Benedict Sterling is in desperate need of money, and his solution is a ball, where a wealthy bride will win the title of countess, even if it does come with one thunderously ill-tempered earl.

Portia agrees to a scheme to help Benedict look like a slightly more appealing bridegroom, while she continues to search for Miranda. Then there's the small matter of the castle, complete with a ghost intent on driving her out. But once Portia sets her mind on something, she's not going anywhere.