My children went through the same stage. What finally worked was immediately grabbing him and firmly saying," Did you hit her? We do NOT hit people!" Make sure you have a VERY angry face. and put him in the high chair. then in front of him, immediately cuddle and speak softly, comforting to his sister. This not only lets her know that you empathize with her pain, but will start to teach him that it really hurts to be hit by someone. At this age, children, especially boys do not have a concept of others feelings or that the world does not literally revolve around them. They need to be taught. Hitting them will only make them angry. He has learned that hitting will cause a reaction, just like pushing a button on a toy, makes it light up, it's just a mechanical reaction to him, and the reaction might be to make her squeal, to get attention from you, or to just make her let go of something he wants. His new lesson if going to be that everytime he does this, he will be placed in the baby seat for three minutes...and the three minutes starts when he is quiet, not kicking and screaming. Get an egg timer and set it for three minutes. Tell him that you will start the time when he is quiet. If he continues to fit, just leave the room...with poor sister. He will be very angry at first, but will learn quickly that he got himself into this mess and he can get out of it. When he is done with three minutes of quiet to think about why he's there. Take him down and hold him, hug him and ask him why he was in time out. You may need to remind him. Tell him that when you hit your sister, it really hurts her, remind him of a time when he was hurt, like a skinned knee or bumped head. Tell him to look at her face, did you see her cry? She was very sad. I know you are SO MAD when she takes your toys. etc. But she is a baby and doesn't know how to share like you do. You need to use your words to tell her no or to leave it alone. If she doesn't listen to you, because she's a little baby, come tell me and I will help you. I will make sure she doesn't take your toys. You don't need to hit her. Just use your words, ok! Then ask him to show you how he could use his words. Tell him to oretend she is taking away his toy, what woudl you say to her? You can offer suggestions, like, "Katie, NO, don't touch my truck...NO Katie, that's mine!" Mommy, she taking my truck! " Tell him mommy will say, Good job using your words Johnny! Katie, no no, you need to leave his toys alone, did you hear his words? You need to listen and not touch his toys, ( then just remove the baby). This will show him that you are his helper and will come to his rescue if he is having a hard time. If he is having bad day, just separate them until he chills out.