We discourage tattletaling around here.
Sometimes the tattlER gets in as much trouble as the tattlEE so that they'll both learn a lesson.
Other times the teller is the ONLY one who gets in trouble, because--quite frankly--I just don't care if your sister is making googly eyes right now or humming the theme to Barney.
Cassie: Daddy! Shelby's...
Daddy: I don't want to hear any tattling.
(a minute passes)
Cassie: Which do you think is louder...Shelby's voice right now or fireworks?
Ooh. A loophole. Does that really count as her telling me her sister is being too loud? I'll need to check some previous case law and get back with you.