I am reluctant to view this behavior as a psychiatric symptom in a child so young. Before assuming that the symptom has a
psychologicalChild neglect and psychological abuse or emotional basis, which it might, it is important to consider medical issues. This order of priority is all the more important to follow if the child is, to all appearances, well adjusted (though the behavior toward his sister lends some doubt to how well adjusted he is). A certain amount of sibling conflict is, of couse, not unusual, but when such conflcit is frequent and is embodied in physical aggression there is more reason to worry. What I would do right away is notify his pediatrician about the symptom and perhaps arrange an evaluation by a pediatric gastroenterologist.