Wicked Deception by Carole Mortimer

Maximillian Smythe, the Earl of Carlton, is the fourth of the eight gentlemen, known as The Sinners, tasked with proving or disproving the guilt of a lady in Society as being a spy for Napoleon, and a traitor to the English Crown. The lady Maxim is to investigate is his young and beautiful widowed stepmother, Lady Heather Smythe, the Dowager Countess of Carlton.
Maxim remembers Heather as being passionate and wild as the rugged Cornish coastline where she resides with her young son. Ralph is Maxim’s half-brother and his heir since their father died eighteen months ago. Heather is far from pleased when her arrogant stepson, Maxim Smythe, arrives at Treganon House for an unexpected visit. She is only too well aware of the contempt Maxim has for her. As she holds less than fond memories of him.

From the moment of Maxim’s arrival, they treat each other with suspicion.
Heather believes Maxim, a friend of the Prince Regent, to be in Cornwall to investigate the smuggling in the area. Something her family has been involved in for several centuries.
Maxim knows Heather is hiding something, but he is unsure if it is her role as a spy for the imprisoned usurper, Napoleon, or something else.
While the turbulent seas roil and crash in the cove beneath Treganon House, the desire and passion Maxim and Heather once shared flares back into life, and this time refuses to be extinguished.