A sibling is one of two or more individuals having one or both parents in common. A full sibling is a first-degree relative. A male sibling is a brother, and a female sibling is a sister. In most societies throughout the world, siblings often grow up together, thereby facilitating the development of strong emotional bonds. The emotional bond between siblings is often complicated and is influenced by factors such as parental treatment, birth order, personality, and personal experiences outside the family.[1]

Identical twins share 100% of their DNA.[2] Full siblings are first-degree relatives and, on average, share 50% of their genes out of those that vary among humans, assuming that the parents share none of those genes.[2] Half-siblings are second-degree relatives and have, on average, a 25% overlap in their human genetic variation.[3]



Two brothers from Haiti
Full siblings

Full siblings (full brothers or full sisters; or brother and sister) have the same biological parents and are 50% related (full siblings share 50% of their genes out of those that vary among humans).[2][3] Identical twins by definition are 100% related.[2]


There are two types of twins: identical and fraternal. Identical twins have exactly the same genes; fraternal twins are no more similar than regular siblings. Often, twins with a close relationship will develop a twin language from infanthood, a language only shared and understood between the two. Studies suggest that identical twins appear to display more twin talk than fraternal twins. At about 3 years of age, twin talk usually ends.[4]

Researchers were interested in subjects who were in the later years of life. They knew that past studies suggested that genetics played a larger role in one's personality in the earlier years of their life. However, they were curious about whether or not this was true later on in life. They gathered subjects with a mean age of 59, who included 99 pairs of identical twins, and 229 pairs of fraternal twins who were all reared apart. They also gathered twins who were reared together: 160 pairs of identical twins, and 212 pairs of fraternal twins. They studied the most heritable traits in regard to personality, which are emotionality, activity level and sociability; also known as EAS. This study found that identical twins resembled each other twice as much as fraternal twins, due to genetic factors. Furthermore, environment influences personality substantially; however, it has little to do with whether they are reared together or apart. This study also suggests that heritability is substantial, but not as substantial as for younger subjects; it has less significance later on in life.[5]



Half-siblings are people who share one parent but not both. They may share the same mother but different fathers (in which case they are known as uterine siblings or maternal half-brothers/half-sisters), or they may have the same father but different mothers (in which case, they are known as agnate siblings or paternal half-brothers/half-sisters. In law, the term consanguine is used in place of agnate). They share only one parent instead of two as full siblings do and are on average 25% related.[3]

Theoretically, there is a chance that they might not share genes. This is very rare and is due to there being a smaller possibility of inheriting the same chromosomes from the shared parent. However, the same is also theoretically possible for full siblings, albeit (comparatively) much less likely.[3]

Half-siblings can have a wide variety of interpersonal relationships, from a bond as close as any full siblings[citation needed], to total strangers.

In law (and especially inheritance law), half-siblings have often been accorded treatment unequal to that of full siblings. 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 siblings of full-blood. Unequal treatment of this type has been wholly abolished in England,[6] but still exists in the U.S. state of Florida.[7]


Three-quarter siblings have one common parent, while their unshared parents have a mean consanguinity of 50%. This means the unshared parents are either siblings or parent and child (similar terminology is used in horse breeding, where it occurs more frequently). Three-quarter siblings are likely to share more genes than half siblings, but fewer than full siblings.


In this case the unshared parents are full siblings. Furthermore, the three-quarter siblings are also first cousins.

In the case where the unshared parents are identical twins, the children share as much genetic material as full siblings do.





In this case, a woman has children with two men who are father and son, or a man has children with two women who are mother and daughter. These children will be three-quarter siblings. Furthermore, the two offspring will have an aunt/uncle-nephew/niece relation. An historical example of this is actress Gloria Grahame. She bore children with her second husband Nicholas Ray, and her fourth husband Anthony Ray, who was Nicholas Ray's son by another marriage.[12]


"Stepsiblings" (stepbrothers or stepsisters) are the children of one's stepparent from a previous relationship. They are unrelated by blood.


"Foster siblings" are children who are raised in the same foster home, foster children of the person's parents, or foster parents' biological children.


Two "adoptive siblings" are raised by a person who is the adoptive parent of one and the adoptive or biological parent of the other. Adoptive siblings are legally related but need not be blood-related or biologically related.


"Sibling cousins" are those who share one parent (as half siblings do) while the unshared parents are themselves siblings or cousins. That is, their fathers are brothers or cousins but they share the same mother, or their mothers are sisters or cousins and they share the same father. This is a broader category than, but inclusive of, the 3/4 sibling above.


One's sibling-in-law is the sibling of one's spouse or the spouse of one's sibling.


One's co-sibling-in-law is one's sibling-in-law's spouse or sibling: One's sibling's spouse's sibling or one's spouse's sibling's spouse.


Godsiblings (godbrothers or godsisters) are the children (biological, step, or adoptive) of one's godparent, or the godchildren of one’s parent; or two or more children who have a common godparent: the other godchildren on one’s godparent. If the godparents are not chosen within the family, they are unrelated by blood.


In cultures with milk kinship, a milk sibling is a person who is not one's biological sibling but was nursed by the same woman as oneself. The concept exists in Islamic law and Jewish law.


Not to be confused with a consanguineous sibling, a blood brother or blood sister is a person to whom one has sworn loyalty through a ritual blood oath. The custom is rare in Western culture.


A Cross Sibling is someone that is related to a person's half sibling/siblings' father or mother through an relationship with another individual after divorcing that person and half siblings' particular parent, there is no relation between a Cross Sibling and the person, but there is one connection however, they both share one or multiple Half-Siblings. Many Cross Siblings are also stepsiblings. Words like "maternal" and "paternal" are typically used to describe which side of the family they are on (maternal cross siblings are one's uterine sibling's half sibling, and paternal cross siblings are one's agnate sibling's half sibling). Terms like "uterine" and "agnate" are less common but also acceptable.


A pseudosibling is someone that is not a sibling but has a relationship like a sibling, or that one considers to be like a sibling. Pseudosiblings are often not related by blood (consanguinity) and are typically very close friends.

Other Languages
العربية: شقيق
asturianu: Hermanu
azərbaycanca: Qardaş
বাংলা: ভাই
български: Братя и сестри
català: Germà
čeština: Sourozenec
chiShona: Hanzvadzi
eesti: Õved
español: Hermano
euskara: Anai-arrebak
فارسی: هم‌نیا
français: Fratrie
Frysk: Sibling
galego: Irmán
한국어: 형제자매
Ido: Frato
Igbo: Nwanna
Bahasa Indonesia: Saudara
italiano: Fratello
עברית: אחאות
қазақша: Апа
Kiswahili: Kaka
la .lojban.: tunba
lumbaart: Fredèl
मराठी: भाऊ
Mìng-dĕ̤ng-ngṳ̄: Hiăng-diê ciā-muói
Nederlands: Sibling
नेपाली: भाइ
日本語: 兄弟姉妹
norsk: Søsken
norsk nynorsk: Sysken
ଓଡ଼ିଆ: ଭାଇ
polski: Rodzeństwo
português: Irmão
русский: Сиблинги
Scots: Siblin
sicilianu: Surastra
Simple English: Sibling
slovenščina: Sorojenec
Soomaaliga: Walaal
svenska: Syskon
Tagalog: Kapatid
татарча/tatarça: Кардәш
తెలుగు: అక్క
Türkçe: Kardeş
українська: Суродженці
Tiếng Việt: Anh chị em
文言: 兄弟
Winaray: Bugtu
ייִדיש: געשוויסטער
粵語: 兄弟姊妹
中文: 兄弟姊妹