The Marriage Obligation by Susana Ellis

Confirmed spinster meets British spy. What could possibly go wrong?

At eighteen years old, Cornelia Hardcastle discovered an ugly family secret that could ruin her family. There is only one way to ensure the world doesn’t learn the truth: never marry. Now that she’s twenty-four, however, her parents have put their foot down. She must marry.

The second son of a viscount, Preston Warrington is happy to leave the viscount business to his brother so he can travel the world in search of adventure. To his shock, he returns home from his service to the Crown as a spy to find his family has decreed that he marry.
The notorious Marriage Maker suggests these two marriage-averse individuals are perfect for each other. Shock sets in—then they realize the genius of his suggestion. Little do their families know their whirlwind romance is a sham, and their marriage is in name only.