Have Military Law Questions? Ask a Military Lawyer.
The present law provides a peacetime exemption for anyone whose parent or sibling was killed in action, died in the line of duty or died later as a result of disease or injury incurred during military service. The service person does not have to be the only son or daughter in order to qualify; if there are 4 son's in the family and one dies in the line of duty, the remaining three would qualify for the surviving son status under the present law.
This provision is applicable only in peacetime, not at times of war or national emergency declared by Congress. The fact that a serviceperson is an only child does not qualify him/her for consideration.
So there is no current prohibition from deploying both of you.
All the best and thanks for your service,