My youngest DD went through this as well and some babies are jsut more inclined to do it than others. I even got asked once if I had done the 123 magic course (which is for parenting difficult children) - I laughed and said it is just a developmental stage, don't you know that? I found that telling her 'no' and then moving her away from the other child worked well and she got the message that it wasn't OK. It can take a while so you need to persevere with her.