avunculate marriage is a marriage with a parent's sibling or with one's sibling's child -- i.e., between an uncle or aunt and their niece or nephew. Such a marriage may occur between biological (consanguine) relatives or between persons related by marriage ( affinity). In some countries, avunculate marriages are prohibited by law, while in others marriages between such biological relatives are both legal and common, though now far less common.
If the partners in an avunculate marriage are biologically related, they normally have the same genetic relationship as half-siblings, or a grandparent and grandchild - that is they share approximately 25% of their genetic material. (They are therefore more closely related than partners in a
marriage between first cousins, in which on average the members share 12.5% of inherited genetic material, but less than that of a marriage between, for instance, cousin-siblings, in which the partners share 37.5% of their inherited genetic material.)
Avunculate marriage is permitted in
Argentina, Australia,  Canada, Finland,  Malaysia,  The Netherlands, and  Russia. It is not permitted in  New Zealand, England and Wales.
Avunculate marriage was the preferred type of union in some pre-modern societies. Marriages between such close relatives were frequent in
Ancient Egypt, at least among members of ruling dynasties.
In societies adhering to
Jewish or Christian faiths, such marriages were sometimes allowed. The Talmud and Maimonides encourage marriages between uncles and nieces, though some early Jewish religious communities, such as the Sadducees, believed that such unions were prohibited by the Torah. Among medieval and especially early-modern  Christians, a marriage between a woman and the sibling of a parent was not always interpreted as violating Leviticus 18; this was especially so among the royal houses of Europe, and in Catholic countries a papal dispensation could be obtained to allow such a marriage.
Such marriages have traditionally been illegal in
Islamic societies and are regarded as prohibited by Islam.
List of historical or mythical avunculate marriages
Perictione and her uncle Pyrilampes
Nahor, son of Terah and his niece, Milcah
Jochebed and her nephew, Amram
Leonidas, King of Sparta and his half-niece, Gorgo 
Alexander I of Epirus and his niece Cleopatra of Macedon
Herod Antipas, Tetrarch of Galilee and Perea and his niece Herodias, Daughter of Aristobulus IV and Berenice (36-37 CE)
Mithridates III of Pontus and his grand-niece, Laodice
Ptolemy VIII Physcon and his niece Cleopatra III of Egypt
Mithridates V of Pontus and his maternal half-aunt, Laodice VI (152 BC)
Ptolemy X Alexander I and his niece, Berenice III of Egypt
Ptolemy XII Auletes and his half-niece, Cleopatra V of Egypt
Emperor Hui of Han and his niece Empress Zhang Yan
Sun Xiu and his niece Empress Zhu Roman Emperor
Claudius and his fourth wife and niece, Agrippina the Younger Byzantine Emperor
Heraclius and his second wife and niece, Martina
Yamato Takeru and his aunt Futaji Irihime
Emperor Jomei and his niece Empress K?gyoku, his aunt Princess Tame
Emperor K?toku and his niece Princess Hashihito
Emperor Tenmu and his niece Empress Jit?, Princess ?ta, Princess ?e, Princess Niitabe
Prince Kusakabe and his aunt Empress Genmei
Emperor Sh?mu and his aunt Empress K?my?
Musa ibn Musa, Wali of Zaragoza and Governor of Upper March and his half-niece Assona Íñiguez
Emperor Junna and his niece Princess Seishi
Emperor Suzaku and his niece Princess Hiroko
Emperor En'y? and his niece Princess Sonshi
Emperor Go-Ichij? and his aunt Fujiwara no Ishi
Emperor Go-Suzaku and his aunt Fujiwara no Yoshiko
Emperor Horikawa and his half aunt Princess Tokushi
Emperor Nij? and his half aunt Princess Yoshiko
Emperor Go-Fukakusa and his aunt Fujiwara no Kimiko Vietnamese Prince
Tran Hung Dao and his consort and paternal aunt, Princess Thien Thanh
Amaury I, Lord of Craon and his half-grandniece, Jeanne des Roches (1212)
Alfonso X of Castile had a concubinage with his paternal half-aunt Maria Alfonso de Leon
John, Constable of Portugal and his half-niece, Isabel of Barcelos (1424)
Afonso V of Portugal and his niece, Joanna of Castile (second wife) (1475)
Jacques of Savoy, Count of Romont and his niece, Marie of Luxembourg, Countess of Vendôme (1484)
Joanna of Naples and her half-nephew, King Ferdinand II of Naples (1496)
Ferdinand II of Aragon and his half-grandniece, Germaine of Foix (second wife) (1505)
Philip II of Spain and his niece, Anna of Austria (fourth wife) (1570)
Charles II, Archduke of Austria and his niece, Maria Anna of Bavaria (1571)
Ferdinand II, Archduke of Austria, and his niece, Anne Juliana Gonzaga (second wife) (1582) Chiefess
Kapohauola and her nephew, Chief Kakaʻe
Maximilian I, Elector of Bavaria and his niece, Archduchess Maria Anna of Austria (1635)
Prince Maurice of Savoy and his niece, Princess Luisa Cristina of Savoy (1642)
Karl Eusebius, Prince of Liechtenstein and his niece, Johanna Beatrix of Dietrichstein (1644)
Philip IV of Spain and his niece, Mariana of Austria (second wife) (1646)
Leopold I, Holy Roman Emperor and his niece, Margaret Theresa of Austria (1666).
Voltaire (François-Marie Arouet), lived in concubinage with his niece, Marie Louise Mignot Denis. 
Prince Augustus Ferdinand of Prussia and his niece Margravine Elisabeth Louise of Brandenburg-Schwedt (1755)
Pedro III of Portugal and his niece Maria I of Portugal (1760)
Prince Benedetto, Duke of Chablais and his half-niece Princess Maria Ana of Savoy (1775)
Infanta Benedita and her nephew, José, Prince of Brazil (1777)
Prince Eugene of Saxe-Hildburghausen and his niece, Caroline of Saxe-Hildburghausen (1778)
King Kamehameha the Great of Hawaiʻi and his niece, Queen Ke?p?olani (c. 1796)
Infante Antonio Pascual of Spain and his niece, Infanta Maria Amalie of Spain (1796)
Sir John Acton, 6th Baronet, Prime Minister of Naples and his niece Marianna Acton (1799)
Francis IV, Duke of Modena, and his niece, Maria Beatrice of Savoy (titular queen of England and Scotland according to the Jacobite succession) (1812)
Leopold, Prince of Salerno and his niece, Archduchess Clementina of Austria (1816)
Infante Carlos, Count of Molina, and his niece, Infanta Maria Francisca of Portugal (1816), and later his niece, Maria Teresa of Portugal (1838)
Kamehameha II and his half-niece Kalani Pauahi
Ferdinand VII of Spain and his niece Maria Isabel of Portugal (1816), and later his niece Maria Christina of the Two Sicilies (1829)
Gustav, Landgrave of Hesse-Homburg and his niece, Princess Louise of Anhalt-Dessau (1818)
Leopold, Grand Duke of Baden and his half-grand niece Princess Sophie of Sweden (1819)
Infante Francisco de Paula of Spain and his niece Princess Luisa Carlotta of Naples and Sicily (1819)
Ernest I, Duke of Saxe-Coburg and Gotha and his niece Duchess Marie of Württemberg (second wife) (1832)
James Mayer de Rothschild, founder of the French branch of the Rothschild banking family, and his niece Betty Salomon von Rothschild (c. 1825).
Prince Francis, Count of Trapani and his niece Archduchess Maria Isabella of Austria (1850)
Mongkut and his half-grandniece Somanass Waddhanawathy (1851), and his half-grandniece Debsirindra (1851), and later his half-grandniece Phannarai (1851).
Svasti Sobhana and his half-niece Abha Barni
Richard von Metternich (son of the famous Austrian Chancellor) and his niece, Pauline von Metternich (1856).
Duke Nicholas of Württemberg and his half-niece Duchess Wilhelmine of Württemberg (1868)
Prince William of Hesse-Philippsthal-Barchfeld and his half-niece Princess Juliane of Bentheim and Steinfurt (1873), and later his half-niece Princess Adelaide of Bentheim and Steinfurt (1879).
Amadeo I of Spain and his niece, Maria Letizia Bonaparte (second wife) (1888)
Henryk Sienkiewicz, Polish novelist, and his niece, Maria Babska. (1904) 
Alois Hitler and his niece Klara Hitler, parents of Adolf Hitler. After they were married, Klara still called her husband " uncle".(1885)  Hitler himself declared that his own half-niece  Geli Raubal was the only woman he ever loved.  Norodom Sihanouk and his half-aunt Sisowath Pongsanmoni (1945), and later his half-aunt Sisowath Monikessan.
". Marriage Act 1961, s 23B"
Pikkanen, Antti (24 July 2014). "Lapsena alttarille - Jenna Karjalainen meni naimisiin alaikäisenä". . Nyt.fi Helsingin Sanomat . Retrieved 2015. [Oikeusm]inisteriö käsittelee myös muita avioliittoon liittyviä poikkeuslupia. Lupaa voi anoa, jos esimerkiksi haluaa mennä naimisiin sisarensa lapsen kanssa. Mutta sellaisia hakemuksia tulee hyvin harvoin, 2000-luvulla pari kolme.
Law Reform (Marriage and Divorce) Act 1976 Archived March 7, 2012, at the Wayback Machine (for Hindus only)
The Family Code of the Russian Federation, Article 14 (in Russian)
"Schedule 2: Forbidden marriages -- Marriage Act 1955 (as of 25 February 2012) -- New Zealand Legislation". Parliamentary Counsel Office. 25 February 2012 . Retrieved 2012. A man/woman may not marry his/her-... (4) father's sister/brother; (5) mother's sister/brother; ... (19) brother's daughter/son; (20) sister's daughter/son
Avunculate Marriage in the Bible
Sparta Revisited - Spartan Leodnidas I and Gorgo
Durant, Will; Ariel Durant (1965). . The Age of Voltaire: a History of Civilization in Western Europe from 1715 to 1756, with Special Emphasis on the Conflict between Religion and Philosophy The Story of Civilization: Part IX. New York: Simon and Schuster. pp. 391-93.
^ See the Polish popflock.com resource article on "
The Hitler Family Tree
Family tree of Adolf Hitler
Shirer, William L. (1960). . New York: Simon & Schuster. The Rise and Fall of the Third Reich ISBN . 978-0-671-62420-0