Taking Aim by Elle James

Former FBI agent Zach Adams understands the danger of tracking a cartel, but he fails to anticipate his desire for the sister of the missing person he's searching for. Determined to find her twin, Jacie Kosart is as strong-willed as she is alluring. But when Zach's ruthless search for the kidnappers lands her in danger, Zach realises he'll risk everything to keep her alive!