The thought occured to me today: Perhaps the brothers really don't know what happened. They may have just driven J and N to the beach and dropped them off. They may have felt pressured to lie to protect J and possibly PVDS because of PVDS's power and connections. And they may feel they need to leave Aruba because they changed their story and were no longer willing to stick to the story J and PVDS asked them to tell. Thus, they may no longer be safe on Aruba. Perhaps their statements can be used in court, even if they themselves are not present.
Just a theory.
In a way, this is part of what I think is wrong with the Dutch system. It is easy for them to cover up crimes because the public never gets to see what evidence if any is collected.
But why would Beth Twitty be so sure they were involved in a violent crime? Maybe she is exaggerating and referring to accessory after the fact? Or does she know that they must have known? They may have cracked in jail to get let free, and now feel they are in danger, as you say.