So, what can be done about it?
It helps if parents schedule the same amount of special time alone with each child every week. However that isn't always possible, and doesn't completely solve the problem. It also helps to praise each child for the things that make them unique and avoid comparing them with each other.
There are lots of books about sibling rivalry and sharing them with the kids and discussing the topic can also help. Sharing family activities that everyone enjoys and emphasizing the concept of family unity can also make a difference. Reading books aloud together even with older kids is a good example of that.
Nothing will solve the problem of sibling rivalry completely, but after the children are grown they are likely to get along well together. Time doesn't heal all emotional wounds, but maturity does heal a lot of them. My brother and I quarreled a lot as kids, but now we are close friends and appreciate the memories we share.