Also, I would be extremely peeved if somebody didn't give me a chance to discipline DD myself first (granted I was right there). I wouldn't be okay with someone just yelling 'NO!' and DD not knowing why or what to do because she wasn't used to that.
Oh definitely! If i am right there i wouldn't want someone else, unless it was DP to say something. But if my back was turned or i was in another room then yes say something.

My mum has at times interferred but its reflex shes so used to growling at her own kids and i can see she feels bad but sometimes that does the trick and whatever DD was doing she's stopped. But whenever mum is here im always fluffing around cleaning lol