Ruthless Promise by Em Petrova

Some things are off-limits for a reason, but when a girl like Meadow is practically begging for punishment, how can Colton say no?

After a special ops mission goes south, Colton returns to his small hometown determined to honor the promise he made when his squadron brother was dying in his arms: to look after the man’s little sister, Meadow.
He didn’t know that promise would mean working on a mountainside ranch, raising cattle while she trains the horses. Or that his new boss would be his friend’s father—a man who doesn’t hide his contempt that Colton caused the death of his only son.
Which is what makes what comes next even harder, because the ranch isn’t helping subdue Colton’s bad boy nature any more than the military did…and while claiming his boss’s daughter for his own seems unconscionable when he’s already taken his boss’s son’s life, Meadow’s relentless teasing sparks Colton’s darkest inclinations yet.