Her Captor by Lindsey Hart

Cameron Spade is used to making it on his own.
He built his entire empire from nothing and earned the reputation of being a black hearted devil for ruthlessly dealing with anyone who got in his way. His only soft spot is his younger sister, Veronica, who is all he has left in this world. Cameron would do anything to protect her … even if it implies striking back by kidnapping the sister of the man blackmailing Veronica.
The deal was fair. Sarah Redden in return for those racy pictures of his sister.
What was supposed to be even easier was keeping his little prisoner captive on his private deserted island. What he never expected was to have a wildcat bent on escaping at each and every turn.
And what was even worst was that he could not resist her.
The more she battled him, the more attracted he was with his fiery little prisoner. But she was so off-limits … he did not sympathize with the enemy, EVER.