I am writing for advice with regards to my 2.5 year old boy-girl twins. They are in general very loving and playful with one another, but there seems to be an increase in the physical aggression of our son towards our daughter in the past few weeks - especially since she started potty training and we have been encouraging her (he is not interested/ready) - we think he feels "left out" or jealous of the attention she gets for this. He goes up to her and either pulls her hair or hits her on the head, and does it repeatedly, even after being put in time out and sternly told what he is doing wrong. My daughter does not usually react aside from crying (i.e. she does not display aggression towards him). It tends to happen shortly after she gets attention for something, but sometimes also happens when we aren't even in the room.
We of course try to treat them equally and encourage both of them for their unique features, and have never compared one to the another. Just not sure what to do about this.... I feel like giving him attention in the form of talking to him/putting him in time out may only be encouraging the behavior further. I don't want to be less vocal with my encouragement of my daughter's potty progress or feel that she may not be as inclined to use the potty.
Any suggestions for more appropriate punishment for my son? Or better ways to react to his behavior?