My daughter rats out her brother all the time! An example for this week: I told my son he could play on his iPad for 5 minutes, and walked away. Exactly five minutes later, my daughter materialized next to me and whispered, "Daddy, it's been five minutes and he's still playing on his iPad!"
He doesn't do it as much to her though... he just tells her to stop playing (or whatever) and she does.
Do your kids rat each other out to you?