The Brothers Hawthorne by Jennifer Lynn Barnes (The Inheritance Games #4):

Four brothers. Two missions. One explosive read. Jennifer Lynn Barnes returns to the world of her #1 bestselling Inheritance Games trilogy, and the stakes have never been higher.  

Grayson Hawthorne was raised as the heir apparent to his billionaire grandfather,

taught from the cradle to put family first. Now the great Tobias Hawthorne is dead and his family disinherited,

but some lessons linger. When Grayson’s half-sisters find themselves in trouble,

he swoops in to do what he does best: take care of the problem—efficiently, effectively, mercilessly.

And without getting bogged down in emotional entanglements.

Jameson Hawthorne is a risk-taker, a sensation-seeker, a player of games.

When his mysterious father appears and asks for a favor, Jameson can’t resist the challenge.