From The Art and Popular Culture Encyclopedia
One's sibling is one's brother or sister, respectively meaning a male or female with whom one shares at least one parent. This is usually taken to mean that the two people are genetically very close, though it is not always necessarily the case, for example one or more siblings may have been adopted by their parents.
In most societies throughout the world, siblings will usually grow up together and spend a good deal of their time during childhood together. This closeness may be marked with the development of strong emotional associations such as love and enmity. The sibling bond is often complicated and is influenced by factors such as parental treatment, birth order, personality, and people and experiences outside the family.