DS (2) has been fine with DD up until now. She is sitting up and is much more animated. He interacts well with her most of the time, giving her toys and laughing with her. However, recently he has started to just push her over and hit her with things all of a sudden. I ignore bad behaviour as much as possible and I find that it works well. BUT, I cannot ignore this. I have tried getting cross with him and moving him away. I have tried just scooping DD up and giving her loads of attention. Neither seem to be having the required effect. He will ask to kiss her (his 'sorry'), so he knows that its wrong, but he just does it again later.
Is it normal? Will it stop? Any advice?