Whether the passengers of the other vehicle will be held accountable for not wearing seat belts will be determined by state laws and if they were cited for not wearing safety belts (if it is against the law in your state). If they were cited for not wearing their seat belts then they would need to pay the fines and take care of any other penalties attached to this violation.

If their injuries are going to go above your bodily injury liability limits then you should contact your insurance agent to find out what will occur next and what steps you will need to take. Without knowing the details of your policy and state laws we are unable to give you exact steps to take while your agent should be able to. If the injuries to the other people are beyond your limits you may need to also seek legal advice.