It is a bit difficult to put kids on the same boat if we are talking about discipline principles. Maybe one time or another, you also had that big brother or big sister that persecuted you while you were growing up. If you had this experience you might have this feeling that a younger kid has that feeling of being the victim. The older ones might have the feeling that they are less significant and do not belong as much because of the younger kids. Maybe the bigger kids feel that they can dominate the young ones to have them live in fear.
Now being a parent myself, it is not easy to sympathize with older siblings who bully their younger sibling. As a parent though, you have to balance things and put the two kids in the same boat. Treat them as the same and do not take sides. Here are some suggestions on how you can handle things positively:
Offer the same choice
Ask if they want to settle things and be in peace or do they want to have a line of choices.
Show some faith
Kids can handle the situation. Let them talk, identify what seems to be the problem, and let them suggest the solutions.
Stop taking sides. Regular chats with the kids will let them know that they are all important to you and they should be there for each other too.
Regular talks with the kids should emphasize that there should be no violence nor abusive language inside the home. Make sure that this rule is clear and implemented, and that they cannot get away if they break it. Start with respect for each other and everything should grow from there.