Deadly Game ~ Christine Feehan

This is the fifth book in the Ghostwalkers series. Ken Norton is assigned to protect a senator from an assassination attempt, he soon learns that another team has also been assigned to protect the senator, only neither team knows of the other, and believes the opposing team is the assassins. Along the way Ken meets Marigold, another Ghostwalker, who also happens to be his twin brother, Jack's, wife's twin sister, that they have been looking for.