Sibling rivalry and sibling fighting are two different things. Many siblings are jealous of each other but never fight with each other specifically because of the rivalry. One may be upset that another broke a favorite toy. Another might be angry because the other one is not doing his or her chores. Rivalry comes from jealousy, and it is hard for parents to ALWAYS treat all their kids the same. You really can’t. Each child is unique and needs a specific type of attention at times. If the children ARE fighting though, they need to be separated and the parents have to give them equal time to explain what happened and why, without rancor and/or prejudice.
It really DOES seem to be normal for most kids to think their parents love the other one better. It’s when they act out as a result of that jealousy, that there is a problem. And parents have to ask themselves – ARE they possibly showing favoritism to one child over the other, consistently? If so, it has to be addressed.