Oh, wanted to quickly add that my 2 and half year old daughter behaved much the same way to her little sister. 98% percent of the time they got along fine, then she would do something like hit her littler sister for no reason. I suppose had she been a boy, she may have reacted more aggressively...
I have not replied here in a long time, but I wanted to say that you should not, obviously, leave him unsupervised to play with his little sister. When I was first reading your story, my first impulse is it must be environmental effect. Do you let him watch violent TV shows, and what not. Do you...
Wow Rittmeister!!
Please provide more details. Did you know this person in a past life, and correctly name, or help find/identify the person's living children?