In the beginning of the book of Genesis, Adam and Eve have just been created as the first human beings- Adam from the dust of the Earth and breath of God and Eve from the rib of Adam, taken while he was in a deep sleep (thus the name "wo-man" given). The two are given the commandment to "be fruitful and multiply", so they follow this commandment by having children. They have three that are mentioned in the Bible: Cain (the oldest), Abel (the middle child), and Seth (the youngest); but only Cain and Abel are relevant to this story. The conflict in this story stems from the fact that the Lord looks upon Abel with satisfaction while he looks upon Cain with disdain. This is because Abel gives the best of his flocks when producing sacrificial offerings to God, but Cain only offers the sick and weak of his flock; this shows that Abel is more devoted to God. Therefore, Cain becomes jealous of Abel and lures him out to the fields and murders him- the first murder in the Bible, supposedly setting the precedent for all future murders. This causes all sorts of trouble for the two which extends throughout the Bible for hundreds of generations. First, Cain is thrown out of the Garden of Eden and is forced to roam the Earth with an eternal curse placed on him and his descendants by God. Also, the more New Testament- relevant consequence is that Jesus does not end up coming from the line of Cain. As the firstborn son, Cain should inherit his father's blessings in at least twice the amount of the rest of his brothers, according to Old Testament traditions. However, since he disobeyed God and was exiled, he was effectively "cut off" from this important lineage and his descendants were cursed; they became a nation which was in constant war with God's chosen people and the nation of Israel. All of these consequences came from a sibling rivalry, which is something most siblings in modern times still struggle with.
Analysis
Sibling rivalry is a very natural part of human existence- seen from the so-called beginning of time, when the first siblings shared a very intense rivalry based on jealousy. My brother and I do not butt heads as much as many other sibling pairs; maybe this is due to his incredible kindness, because I surely cannot claim that it is due to my agreeableness. If anything, this is in spite of my incredible stubbornness. There are times, however, when we butt heads- not to the degree of Cain and Abel, but only because it never escalated to one of us murdering the other. Disagreement is disagreement and our situations are seemingly mirrors of each other.