Estate battles have been going on between siblings for as long as there have been estates and wealthy people who pass away. History books are full of the wars that occurred when a king passed away and his children fought with large armies for the crown.
In the U.S., the participants in a family estate fight no longer enlist armies and fight it out on a field of battle. People now fight things out in court. However, the reasons for the disputes today are very similar to what they were hundreds of years ago.
While ostensibly these feuds are over the wealth, crown or other inheritance, there is normally something deeper going on as Psych Central points out in "Death, Wealth, and the Psychological Anatomy of a Family Dispute."
Psychologists and even pop culture have known for a long time about sibling rivalry. Although there is disagreement about the core reasons for the phenomenon, no one disputes that it exists. Whether it has its roots in birth order, a desire to be loved more by parents or something else, sibling rivalry is a real thing that does not go away when children grow up.
The same psychological reasons that lead siblings to argue over who gets to ride shotgun in the car, are the same reasons that lead to arguments over estates.
Little can be done to end sibling rivalry in a family. On the other hand, knowing that it exists and making estate plans with it in mind, can go a long way toward making sure it does not get completely out of control in a fight over an estate.
Reference: Psych Central (Dec. 13, 2016) "Death, Wealth, and the Psychological Anatomy of a Family Dispute."