If you've already tried explaining to your kids that they need to be nice to each other and you haven't seen results, you can you discipline for all of them. If you catch the older ones being mean, apply the 2X-Rule (see Raise Your Kids without Raising Your Voice for a full explanation). Let them know that this behavior will no longer be tolerated. Each time you see them behaving in an unacceptable manner, they will get a consequence (tell them exactly what consequence they will get). Make sure the consequence is a true deterrent. Do the same for the youngster who responds "shut up." Tell him that you will no longer tolerate this response - you have already given him other options. Even though he is being provoked, he has options as to how to respond. People provoke us through our lives and in every relationship - at home, in the community and at work. We need how to handle provocation in a way that will minimize conflict and preserve our relationships. Saying "shut up" is not one of those ways. Moreover, provocation is not an excuse for bad behavior. If it were, we could all go out and murder people who bother us! We have self-control and we must use it. In fact, teaching your 11yr old that he will always have self-control is very empowering. No matter how other people behave (i.e. his future wife and future kids!) he will be able to choose his response. Therefore, if he chooses to say "shut up" then he will suffer a negative consequence. Again, tell him exactly what consequence.  Sarah Chana