People who come from abusive families, especially incestuous families, do not develop normally. Their development is compromised. Dr. Ann Burgess (yes, the Dr. Ann Burgess) did an excellent job of explaining this in the first trial. In addition to being an FBI criminal profiler and a psychiatric nurse who specializes in treating CSA victims, she is also a crime scene analyst. She examined the crime scene and determined that it was a unplanned killing.
Being an an adult under the law does not necessarily mean that one is an adult mentally, psychologically or emotionally. Both brothers were determined to be emotionally younger than their chronological ages - 8 and 10 years old respectively.