how challenging hun. Could you bring it up with your sister in a non-threatening non-dobbing way? More like, "the kids were doing xyz and I wasn't sure if that was ok? what do you normally do?" See how she replies, what their strategies are for discipline and if you are on the same age, maybe next time you could sit them down and lay down the rules beforehand and let them know that not listening etc was unacceptable but you have tlkaed to mum and know that XYZ will be in place if it happens again? I have talked a lot in XYZ and not if any of it makes sense!!