I'm not saying this just to be contrary but, if both JP's parents worked when the kids were young... something could have happened in a babysitter's home or a child care center or a relative's home, etc. Also, I'm still waiting for confirmation that the kids were placed in foster care and, if so, something could have happened there, too. I'm just seeing JP's family as victims at this point because heredity and congenital anomalies diagnosed or not, can lead to predilections in behavior and is also a possibility imo. I'm especially reluctant to try and place any responsibility on the parents when I don't know anything about the family life; as in, "growing up in the P home."
ETA: Siblings don't, strictly speaking, "grow up in the same environment." Home environments change according to circumstances within the family and from without. Example would be financial burdens may become more influential as more children are born; causing changes in parental character. First child might be planned and dearly loved and fussed over... sibling #3 might be born into a tension filled atmosphere with lots of arguing going on. There are countless variables involved, imo.
JMO