Kiss Me, My Duke by Fenna Edgewood

Lost: One duke’s sister. Found: The unexpected love of a lifetime.

Edmund “Dare” Blakeley, the Duke of Dareford, is on a mission to find his younger sister who has been ruthlessly abducted. The only problem? He needs the help of his housekeeper to do it.
Before she was his servant, she had another name… and all of London swooning at her feet.
Miss Laurel Spencer has no wish to cavort with a duke–even if he is her brother-in-law. In her younger days, she saw how love can utterly destroy a woman.
But when the duke’s younger sister and Laurel’s elder brother go missing, she reluctantly agrees to accompany Dare to London to search for them.
As Laurel calls upon old friends in low places, including the most notorious gaming hell in London, her past begins to resurrect itself. Soon Dare is forced to realize how little he knows about the enigmatic woman by his side… and how desperate he is to uncover everything he can about her.
One reckless moment of passion…
Suddenly, a missing sister is the least of Dare’s worries. After a wild night in Laurel’s bed, his happiness hinges on proving his love is sincere… to a woman terrified of ever risking her heart, even if it means becoming his duchess.
As the Blakeley family mysteries are uncovered, not everyone is glad to see the truth coming to light… and one man will do anything to see that it stays buried.