One study found that the age group 10 to 15 reported the highest level of competition between siblings.Sibling rivalry can continue into adulthood, and sibling relationships can change dramatically over the years. Events, such as a parent’s illness, may bring siblings closer together, whereas marriage may drive them apart, particularly if the in-law relationship is strained. Approximately one-third of adults describe their relationship with siblings as rivalrous or distant. However, rivalry often lessens over time. At least 80 percent of siblings over the age of 60 enjoy close ties. Causes
According to Kyla Boyse from the University of Michigan, each child in a family competes to define who it is as an individual and wants to show that it is separate from its siblings. Children may feel they are getting unequal amounts of their parents’ attention, discipline, and responsiveness. Children fight most in families where there is neither any understanding that fighting is not an acceptable way to resolve conflicts nor any alternative way of handling such conflicts; in families in which physical fighting is forbidden but no method of non-physical conflict resolution (e.g., verbal argument) is permitted, the conversion and accumulation of everyday disputes into long-simmering hostilities can have an effect nearly as corrosive. Stress in the parents’ and children’s lives can create more conflict and increase sibling rivalry. Some research indicates that changes in the weather can increase the likelihood of children exhibiting deviant behavior.
Esau was jealous of his brother Jacobs inheritance and blessing; sisters Leah and Rachel compete for the love of Jacob; Josephs brothers are so jealous that they effectively sell him into slavery. These stories are also present in the Quran. Another example is provided in the myth of Romulus and Remus, wherein twin brothers Romulus and Remus fight for control over Rome, and which ends with the death of Remus at the hands of his brother. In folk and fairy tales
Sibling rivalry, bitter jealousy, and envy are notable in several fairy tales around the world. In some tales, the jealousy escalates to outright murder of the successful sibling. Some tale types, according to the Aarne-Thompson-Uther Index, heavily feature sibling and step-sibling rivalry as part of the plot:

ATU 301, "The Three Stolen Princesses" or "Jean de lOurs": a hero, born with strong powers, descends to an underworld realm to rescue three princesses. He sends the three maidens to the upper realm with the help of his brothers (or companions), but they cut the rope and trap him in the subterranean realm. ATU 361, "Bearskin": a man makes a deal with devil: must not pray or bathe for a certain number of years. Later, the man meets a father with his three daughters; only the youngest sister promises to marry the man. After the time is up, the man grooms himself and goes to meet his bride, explaining the whole story. Her sisters seethe with rage and explode with jealousy; the devil takes two souls, instead of one.