Decidedly Off Limits by Stina Lindenblatt

Best Friend Rule #1: never want the guy’s sister.
Oops.

Trent Salway is one of San Francisco’s hot up-and-coming-under-thirty bachelors. A highly successful mutual fund portfolio manager, a workaholic, and a ladies’ man, he’s got everything he could possibly want…except one thing.
Physical therapist Kelsey Quaid dumped her ex-fiancé six months ago because she was tired of not even being a distant second to his career. Which is why despite secretly crushing on her big brother’s best friend for as long as she can remember, there’s no way Kelsey can go there. One workaholic in her life is more than enough, thank you very much.

Before Kelsey’s brother leaves on his Navy SEAL mission, he asks Trent one simple request: make sure Kelsey isn’t dating any assholes while he’s away. As Trent does his best to do just this, he and Kelsey spend more and more time together. Sparks fly, as do a few items of clothing, and the next thing they know things are getting physical between them. Now, Trent has to decide what to do about the woman he’s falling for–the one whose brother would take him down faster than he takes down the enemy. And Kelsey will have to decide if Trent is just another workaholic or if he’s decidedly off limits.