Incest is sexual intercourse between close relatives that is illegal in the jurisdiction where it takes place and/or is socially taboo and can be illegal or legal depending on the jurisdiction. The exact definition, including the nature of the relationship between persons, and the types of sexual activity, vary by country, and by even individual states or provinces within a country. These laws can also extend to marriage between said individuals.
In Australia marriage between an ancestor and descendant or between a brother and sister (including siblings of half-blood), is not permitted and these "prohibited relationships" include relationships traced through adoption. Moreover, incest is a crime in every Australian state and self-governing territory, but definitions and penalties vary.
In all states and territories the legal definition of incest covers sex, whether heterosexual or homosexual, between a parent and child, as well as between siblings (including half-brothers and half-sisters). Whether the definition of incest extends to sex between a stepparent and stepchild or adoptive relationships varies from state to state.
In all jurisdictions except South Australia, the definition also includes sex between a grandparent and a grandchild, and in all other jurisdictions except New South Wales it also covers sex between a "lineal ancestor" and a "lineal descendant", which would include the relationship of great-grandparent/great-grandchild and beyond. Only in Queensland is incest defined to include sex between an uncle or aunt and a niece or nephew, although even here its application would appear to be curtailed by the effect of federal marriage law, as the Queensland Criminal Code itself provides that the crime of incest does not apply to "persons who are lawfully married or entitled to be lawfully married" and Australian marriage law permits (heterosexual) marriage between an aunt or uncle and a nephew or niece, provided both parties are of marriageable age (currently 18). In New South Wales the crime of incest generally only applies where the victim is aged 16 or over (the age of consent in that state); in cases where the victim is under 16, the accused would generally be charged with sexual intercourse with a child under the age of 16, or in cases where the victim is under 10, the accused would generally be charged with sexual intercourse with a child under the age of 10.
In all other jurisdictions the crime of incest also exists where the victim is below the age of consent, but this does not exclude the possibility of bringing the more general charge of sexual intercourse with a child under the age of 10 or 16 as the case may be. This is particularly relevant where a certain form of sexual conduct between related persons falls outside of the legal definition of incest in a particular jurisdiction.
In no Australian state or territory is consent a defence to the crime of incest. A conviction for incest attracts a maximum penalty of 10 years imprisonment in South Australia, 20 years imprisonment in Western Australia and the Australian Capital Territory, 25 years imprisonment in the Northern Territory, Victoria, Tasmania and New South Wales, and life imprisonment in Queensland.
Two or more convictions for incest also places the offender on the Sex Offenders Register for the remainder of their life.
In Brazil, incest is considered any kind of sexual interaction between two blood related humans being. It has no criminal punishment if the involved are over the age of 14, capable of acting upon their legal rights, and that consent means that the relationship is absent of any kind of coercion or fraud. An uncle or aunt is allowed to have a relationship with a nephew or niece provided that they have a health check. 
Under Canadian law, incest is defined as having a sexual relationship with a sibling (including half-sibling), child/parent or grandchild/grandparent while knowing the existence of the blood relationship. It is punishable by up to 14 years imprisonment.
England and Wales
Incest is illegal in England and Wales. It is defined as sex, whether heterosexual or homosexual, between a person and their parent, grandparent, child, grandchild, sibling, half-sibling, uncle, aunt, nephew or niece. It is punishable with up to 14 years imprisonment.
In Finland, the law doesn't specifically mention incest as a word at all, however, marriage between one's sibling, half-sibling, ancestor or descendant is forbidden. Also, sexual acts between one's sibling (but not half-sibling), ancestor or descendant is punishable from a fine up to 2 years in gaol for "sexual act between close relative" unless the person in question is under 18 years old or have been forced or illegally persuaded to perform the sexual act.
In Germany, incest is punishable by law if consummated between people related by blood in direct line only, therefore between parents, grandparents, great-grandparents and their children, grandchildren, great-grandchildren plus among siblings and halfsiblings. The penalty is a fine or up to 3 years of prison. Incest between relatives who are minors (below 18 years old) at the time of offence is not punishable but remains a crime, therefore aiding and abetting of incest between related minors is punishable de:Inzest#Rechtslage in Deutschland. The legal term used in German jurisdiction is "Beischlaf" (engl. coitus), only vaginal intercourse is punishable, other forms of sexual activity remain exempt from punishment.
Regarding marriage, the same rules apply and prohibit marriage between aforementioned relatives .
The criminal liability of incest among consenting adults is socially disputed in Germany, though the Federal Court of Justice (comparable to US Supreme Court) ruled in 02.26.2008  that § 173 StGB  is constitutional in a 7:1 vote with one judge having a dissenting opinion regarding the commensurability.
See also: the case of Patrick Stuebing
Incest is a prosecutable offense in India by law.
Republic of Ireland
Incest is illegal in New Zealand. This applies to parent/child relationships, grandparent/grandchild relationships, and relationships between siblings and half-siblings. A conviction for incest attracts a maximum penalty of 10 years imprisonment.
In Poland, incest is defined in Article 201 of the Penal Code as sexual intercourse with an ancestor, descendant, guardian, ward, brother, or sister, and is punishable by imprisonment for no less than 3 months and no more than 5 years.
Incest is not specifically prohibited under Portuguese law.
In Russia, consensual sex between adults, including incest, is not a crime. However, under the Family Code of Russia, persons who are related lineally, siblings, half-siblings, and a stepparent and a stepchild may not marry.
Incest with a descendant or a full sibling is prohibited by law in Sweden. Half-siblings can marry, but require special approval by the government.
Article 213 of the Swiss Penal Code prohibits incest. Intercourse among siblings or other persons related by blood in direct line is punishable by up to three years imprisonment. The federal government proposed to abolish this prohibition in 2010, arguing that in the few cases where persons were convicted of incest (about three to four each year), other sexual crimes such as child sexual abuse were also committed.
In the United States, every state and the District of Columbia have some form of codified incest prohibition. However, individual statutes vary widely. Rhode Island repealed its criminal incest statute in 1989, Ohio only targets parental figures, and New Jersey does not apply any penalties when both parties are 18 years of age or older. Massachusetts issues a penalty of up to 20 years' imprisonment for those engaging in sexual activities with relatives closer than first cousins and Hawaii up to 5 years in jail for "sexual penetration" with certain blood relatives and in-laws.
In all states, close blood-relatives that fall under the incest statutes include father, mother, grandfather, grandmother, brother, sister, aunt, uncle, niece, nephew, and in some states, first cousins. Many states also apply incest laws to non-blood relations including stepparents, step-siblings, and in-laws.
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- "Marriage Act 1961 s 5". http://www.austlii.edu.au/au/legis/cth/consol_act/ma196185/s5.html#marriage.
- "Marriage Act 1961 s 23B". http://www.austlii.edu.au/au/legis/cth/consol_act/ma196185/s23b.html.
- See NSW: "Crimes Act 1900 s 78A". http://www.austlii.edu.au/au/legis/nsw/consol_act/ca190082/s78a.html.; Vic: "Crimes Act 1958 s 44". http://www.austlii.edu.au/au/legis/vic/consol_act/ca195882/s44.html.; Qld: "Criminal Code s 222". http://www.austlii.edu.au/au/legis/qld/consol_act/cc189994/s222.html.; SA: "Criminal Law Consolidation Act 1935 s 72". http://www.austlii.edu.au/au/legis/sa/consol_act/clca1935262/s72.html.; WA: "Criminal Code s 329". http://www.austlii.edu.au/au/legis/wa/consol_act/cc94/s329.html.; Tas: "Criminal Code s 133". http://www.thelaw.tas.gov.au/tocview/content.w3p;cond=;doc_id=69%2B%2B1924%2BJS1%40GS133%40EN%2B20071018000000;histon=;prompt=;rec=;term=.; ACT: "Crimes Act 1900 s 62". http://www.austlii.edu.au/au/legis/act/consol_act/ca190082/s62.html.; NT: "Criminal Code s 134". http://www.austlii.edu.au/au/legis/nt/consol_act/cc94/s134.html..
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- Family Code of Russia, article 14
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- Template:Cite swiss law
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