It is inevitable when you have more than one child. Eventually, no matter how well they get along most of the time, there will be a fight. In some families, this fighting is rare - in most families, sibling fighting is one of the great energy drains for parents. Kids fight for different reasons and therefore different tactics apply.
Kids fight when they are bored.
Have an anti-boredom plan available at the first sign of boredom fighting. One good answer is to send the combatants into neutral corners with a good book think of it as time out with educational entertainment. You will be amazed at how fast that encourages cooperative play - and improves reading skills at the same time. Kids fight to get your attention. This is similar to the boredom fight.