One of the most annoying things for parents to deal with is the constant battles that break out between their children. And no matter how old they get, this phase just never seems to pass. It can be like living in a war zone. And the words "It's not fair..." become the most hated thing you have to listen to throughout the day.
Set an example: Be a good role model: If you have the habit of negating everything that comes out of your spouse's mouth then you must realize this is a big problem. Nobody gets along perfectly, but the kids don't need to witness your disagreements. They should see you working together towards a solution calmly. No loud voices! And you really need to listen to each other. Kids will learn to bully a less vocal sibling, it's so much easier to blame everything that goes wrong on the one who can't defend himself. Always be polite and soft spoken. Talk about how you feel when things go wrong and what is the best way to react to keep things from getting worse.
Let them settle the problem themselves: You can't jump into every disagreement they have. It often just leads to more bickering and the dreaded "You always take his/her side!" Disagreement on a T.V. program? Tell them if they can't agree the T.V. will be turned off. And don't just give empty threats, follow through! They need to be able to agree on how problems can be settled by finding a solution that suits all of them. "You wear the pink sweater today, I'll wear it tomorrow." And give praise or a small reward to the one who was willing to make the sacrifice first. It will motivate the rest of them.
Sometimes you do need to jump in! If one sibling is a bully you really need to take things into your own hands. Sitting down together and talking it through helps, maybe not immediately but eventually. You need to keep your patience though! And sometimes there will have to be consequences. You will have to make it clear to the other children that there must be absolutely no teasing for the one getting punished, remind them it might be their turn next. The loss of a certain privilege for some time usually works. No ps3, xbox or ipod. Maybe no going out on Friday night with friends.
Rewards for good behaviour: Praise and little rewards for good behaviour show your appreciation. Keep the material things to a minimum. Baking something together, a trip to a favorite place or maybe even just a big hug works wonder for young kids. For older kids maybe certain freedoms? Attending a sleepover, going to a movie with friends whatever interests them within limits of course.
Over time it will become a routine thing and they will be able to settle things on their own a little more peacefully.