ISIS claimed responsibility for the attack, which came just weeks after President Trump said "we have beaten them and we have beaten them badly."
"I just didn't know what to do. Like, I didn't know if this was normal," Tracy Sampson told NBC in her first on-camera interview.
The cousin cannot be charged because Texas state law does not allow charges for anyone younger than 10.
The Department of Defense said Wednesday that the fourth person not yet identified was a contractor.
Prosecutors say 33-year-old Lamar Davenport stabbed his girlfriend E'Dena Hines multiple times in August 2015 while under the influence of alcohol and drugs.