The parents have to discipline themselves in order to discipline their children. Parents who take sides in sibling rivalry encourage jealousy and hate. They should not prefer boys over girls, fair complexions over dark ones, the honor roll over an average student. Neither party can influence the other unless they communicate. Find a time and place to talk to your children.
Children are sometimes in a 'bad mood' upon returning from school, loaded with homework.
The best time to have a chat is during breakfast and evening dinner. When you do argue, do it patiently, one person speaking at a time. Be specific and separate emotion from facts. Speak in a low tone, Screaming decreases the intake of the message. Practice active listening from each other's view, even if you don't agree.