Sibling rivalry can develop for many reasons. In some cases it's due to the personalities of the children, but then other times children may feel jealous, for example, when they feel that a sibling gets more love or privileges than they do. But no matter why it happens, it still needs to be dealt with so that everyone can get along.
If the rivalry takes the form of physical fighting between the children, it is very important for children to know that there are "no hurting" rules, as opposed to just saying, "no pinching" or "no grabbing." Let them know right away that you won't tolerate that behaviour by saying, "we don't hurt anyone in this family."
If the children are arguing constantly, letting them work things out on their own is good in many cases. But be ready to step in when these little arguments start turning into long-standing issues. New research shows that children can suffer immensely if verbal taunts and threats by brothers and sisters go on and on.
To keep things peaceful, try to give each child one-on-one attention at least part of each day. This will make each child feel that she is still special to you in your eyes.
Finally, when jealousy rears its ugly head, it's important not to blame one child or the other. Encourage the children to talk about their feelings of envy and jealousy. It's not going to be easy, but try to stay calm and listen to what they have to say in these situations. Try to emphasize the strengths of each individual child.