Lord of Mistrust by Elizabeth Keysian

Following their hearts could destroy the monarchy.
Headstrong Chloe dresses as a boy and runs away to her birth mother to escape a horrendous marriage. She’s shocked to discover that her parent owns a bawdy house, and is in no position to help- nor will she reveal the identity of Chloe’s father. When a street accident throws Chloe into the lap of the tempting Robert Mallory, he offers distraction and adventure, but his stubborn refusal to trust her endangers them both.

Hot-headed Robert Mallory is battling to protect his sister, his livelihood, and his honor. He’s a spy who can’t follow the rules and distrusts everyone, particularly the delectable young woman from the bordello. Having endangered her, then rescued her from a nest of traitors, he learns that Chloe is the natural daughter of the one man he can’t afford to upset, Sir Mortimer Fowler. Offering marriage to save Chloe’s reputation is out of the question, as Fowler needs her for bait in a deadly trap.

Robert is faced with an impossible choice. He’s desperate to save Chloe, but if he follows his heart, the security of the entire realm is at risk.