As floodwaters tore through Texas Hill Country, 14-year-old twins Clara and Nolan Shelton became unexpected heroes. Separated at different summer camps, each faced terrifying scenes of devastation. Clara was at Camp Mystic. With medical staff overwhelmed, she rushed to the infirmary to care for injured campers. Nolan, at a nearby boys’ camp, helped carry younger children to higher ground—while alligator...