According to current regulations and directives, being an only son/daughter does not constitute sole surviving son/daughter status with regard to service in the Armed Forces. The fact that a son/daughter is the only child or son/daughter does not exempt him from serving overseas or in a combat zone. They must be the survivor of one who died as a result of military service to qualify.
Present law provides a peacetime exemption from assignment to a combat zone to anyone whose parent or sibling was killed in action, died in the line of duty, or dies later as a result of disease or injury incurred in the line of duty while serving in the Armed Forces of the United States. Also exempted are those whose parent or sibling is in a captured or missing status as a result of service in the Armed Forces during any period of time. If the Marine's father or mother is permanently 100% physically disabled (to include 100% percent mental disability), as determined by the Department of Veteran's Affairs or one of the military services and is hospitalized on a continuing basis and not gainfully employed because of the disability, he or she also qualifies for the exemption.
It is important to keep in mind that the above provisions are directly related to service connected deaths. Further, an individual does not have to be the only surviving son/daughter in order to qualify for surviving son/daughter status. If there were four sons/daughters in a family and one dies in the line of duty, the remaining three would qualify for surviving son/daughter or brother/daughters status under the present law.