Undercover Billionaire by Erin Swann

The undercover assignment sounded simple, until I learned who I’d be protecting.

Kelly:
The note froze my blood. It threatened to make me the next to die if I didn’t cooperate. When I asked for protection, I had no idea the FBI would send a Cartwright, one of my family’s mortal enemies, to be my bodyguard.
The Cartwrights have been at war with us forever. The plan is he’ll pretend to be my boyfriend. No, that won’t be awkward at all. My family would kill me if they found out.
It turns out the man I’m sharing my house with isn’t the monster I expected. I shouldn’t be falling for Adam, yet the more time I spend with him, the stronger his pull becomes.
How can I possibly choose between Adam and my family?

Adam:
With a ton of zeros on my bank balance, I joined the Bureau to make a difference, not a living.
When we learn of the latest threat from, SMK, the Smithsonian Museum Killer, catching him becomes a top priority in the office. I get assigned as undercover protection. The surprise comes when I learn her name: Kelly Benson.
The Hatfields and the McCoys were on better terms than our two families. Years ago, her brother ruined my future, and now I’m tasked to keep his little sister safe. Life isn’t fair.
Uncomfortable revelations from our pasts soon complicate everything. Before long, I only know one thing for sure. I don’t want this assignment to end.
Forced into close quarters and constant contact, mutual attraction soon ignites. The two must reexamine what they’ve believed for years about each other. When old truths are questioned, family dynamics become as volatile as the killer stalking Kelly.