My two little ones seem to derive a certain satisfaction at seeing the other one being punished.
My girl is always telling me to "beat baby" because "baby naughty". My boy has caught on too. The other day he repeatedly said "Mummy sco" (scold), "Daddy sco" (scold) while pointing to his sister.
Recently Mr MG caught them doing something naughty (I don't remember what and I don't know who was the culprit.) When Mr MG asked baby "Did you do this?" he said "Cheh cheh" (sister). Mr MG then called my girl to the room and asked her the same question "Did you do this?" to which she replied "Baby" and they both pointed their fingers and one another! (*shakes head*)
Sometimes they are the best of friends though, chasing each other around the house, clowning around and screaming with laughter. When I hear their squeals of joyful laughter, I feel that only a sibling could give this kind of silly play that a parent can't quite match or at least I can't match that kind of play since I don't have that kind of energy they seem to have.