Federal law allows a "peacetime" draft exception for individuals who have had an immediate family member (father, mother, brother, or sister) die or become 100 percent disabled as a result of military service. Note the law does not require the person to be the "last" in their line. This exemption only applies to peacetime (and we don't have a peacetime draft right now, anyway), and not to periods of war or national emergency declared by Congress.
Additionally, the Department of Defense does allow a military member who has an immediate family member who dies on active duty, or becomes 100 percent disabled, or becomes a POW, to request a voluntary discharge. Again, note that the member doesn't have to be the "sole survivor." This program is also not applicable during times of war or national emergency declared by Congress. Also note it's a voluntary program. The military member must apply for it. A military member who enlists or re-enlists after being notified of the death/disability (due to military service) of their immediate family member is considered to have waived their status under this program.
Finally, each of the services have regulations which exempt "surviving sons/daughters" from serving in combat zones. Again, the program is voluntary, and again, they must have an immediate family member who has died on active duty, 100 percent service-connected disabled, or in POW status. Simply being the "last in the line" doesn't count.
For complete details, see our Sole Surviving Son/Daughter article.