A sibling is a brother or sister; that is, one who shares the same parents.

In most societies throughout the world, siblings usually grow up together and spend a good deal of their childhood with each other. This genetic and physical closeness may be marked by the development of strong emotional associations such as love or 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.

Full sibling

A full sibling (full brother or full sister), is a sibling that shares both biological or adoptive parents.

Half sibling

A half sibling (half brother or half sister) is a sibling with one shared biological or adoptive parent. A half sibling that shares the same mother (but different fathers) is known as a uterine sibling, whereas one that shares the same father is known as an agnate sibling. In law, the term consanguine is used in place of agnate. Half siblings can have a wide variety of interpersonal relationships, from a bond as close as any full siblings, to total strangers.

At law (and especially inheritance law) half siblings were often accorded unequal treatment. Old English common law at one time incorporated inequalities into the laws of intestate succession, with half siblings taking only half as much property of their intestate siblings' estates as other siblings of full-blood. Unequal treatment of this type has been wholly abolished in England and throughout the United States.

Stepsibling

A stepsibling (stepbrother or stepsister) is a sibling with whom an individual bears no blood or equivalent adoptive relation, and is related by the marriage or relationship of one parent of the individual to one parent of the sibling.

Siblings through breast feeding

Milk brothers or sisters are children breastfed by a woman other than their biological mother, a practice known as wetnursing and once widespread in the developed world, as it still is in parts of the developing world.

In Islam those who are fed in this way become siblings to the biological children of their wetnurse, provided that they are less than two years old and have been breastfed five times or more by her. Islamic law (shariah) codifies the relationship between these people, and certain specified relatives, as rada; once they are adult, they are mahram, meaning that they are not allowed to marry each other, and the rules of modesty known as hijab are relaxed, as with other family members.