twin toddler agression at the other

Hi, I have twins. I know this may not necessarily be the right board, but they are toddlers 23m old. If I get mad at my daughter (e.g. this morning she was pulling things off the countertop and I sort of barked 'stop that'), she ends up pinching or hitting her twin. He was standing nearby, had nothing to do with anything, and she always takes out her aggression on him. It's not like I don't get upset at him too when he does something he shouldn't do. but he doesn't go an hit his sister or take it out on her. It's not a one time issues, it's if I get upset with her she always does that. Why does she do that? is that normal? Do you think maybe she thinks I favor him, and that's why she does it? I feel terrible about it, and I wish she knew I loved her, even if I get upset at her, but she doesn't seem to understand it. Sometimes I do feel like I am nicer to my son, but that's only because he's a happier go lucky toddler, and she's really moody. She has always been a moody baby, had colic, is difficult to console, cries a lot harder and for longer time than my son. I'm really frustrated with myself too that this has been going on for 2 years.