Avunculate marriage

An 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 Norway, Chile, Argentina, Australia, Canada, Finland, Malaysia, The Netherlands, and Russia. In the United States it is permitted in some circumstances in two states. In New York a marriage between a woman and her mother's half-brother was upheld by the New York Court of Appeals. In Rhode Island there is an exception to the general prohibition against "kindred marriages" for Jewish marriages allowed by that religion. 
It is not permitted in New Zealand, England
History.
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.
Avunculate marriages were prominent in the House of Habsburg. For example, Charles II of Spain was the son of an uncle and niece, Philip IV and Mariana of Austria; in turn, both of Philip's parents (and therefore both of Mariana's maternal grandparents) were the children of uncle-niece marriages, one of which also produced Mariana's paternal grandfather. As a result, instead of Charles' parents, grandparents, great-grandparents, and great-great-grandparents adding up to 30 different individuals, they numbered only 23.