There are many reasons as to why siblings fight. They may feel jealous of the sibling for getting more parental attention. Our approach to each child varies because each child is different after all. Some are independent and responsible from a very young age while the other one takes a lot of time and need you less. But this can be interpreted differently and they feel that you love him less than the other child.
Sometimes the elder child gets bossy and tries to take the younger ones toys, clothes and other possessions and you knowingly or unknowingly take his side. And hence the other one feels cheated.
When kids are bored, tired and frustrated they vents out their frustrations by fighting with each other. You as parents should be very careful in front of them and should not vent out your anger on them by shouting, banging doors and using bad words. If you do that on a regular basis then they will pick up unhealthy ways of fighting with each other.
As parents when kids are fighting with each other you should stay away from their arena. Let them fight their own battles. Let them sort out themselves. This approach also lets them learn to resolve conflicts. But this does not again mean that you will allow them to bash each other up. When you see that they are hurting each other then it is time to step in. don’t start screaming at them the moment you step in, just send them to separate spaces and when they calm down call them and make them understand that they cannot keep fighting with each other . In fact teach them to discuss their problems with each other in a sober way.
Teach them anger busters like counting ten when they angry. Anger busters are things that they can do when they start getting angry. Tell them to think about the happy times that they share with each other instead of getting angry and fighting with each other.
Do spend time together as a family. You can play a sport together, or some indoor games, bake a cake together or else go for picnic or go for a nature walk together. This way they will have fun together and will not crave for attention from parents.