Complex sentence: To increase their security, they used Sea Rogue as their decoy, and to make sure that he performed their bidding, they took his uncle hostage and threatened to sink him if Sea Rogue disobeyed them.
Split into simpler sentences: To increase their security, they used Sea Rogue as their decoy. To make sure that he performed their bidding, they took Sea Rogue's uncle hostage and threatened to sink him if he disobeyed them.