Levirate marriage is a type of marriage in which a woman marries one of her husband's brothers after her husband's death, if there were no children, in order to continue the line of the dead husband. The term is a derivative of the Latin word levir, meaning "husband's brother". Levirate marriage has been practiced by societies with a strong clan structure in which exogamous marriage, i.e. that outside the clan, was forbidden. It is or was known in societies including the Punjabis, Jats, Israelites, Huns (Chinese "Xiongnu", "Hsiong-nu", etc.), Apache, Mongols, and Tibetans.
In Judaism, levirate marriage, known as yibbum, is a marital union mandated by the Torah in Deuteronomy 25:5-10, obliging a brother to marry the widow of his childless deceased brother. This was practiced because children were extremely important to the Israelites as well other ancient near east societies. Having children led to security and was a sign of status. Without children there was no one to inherit the family's land which was considered very valuable since it was given to them by God. A barren woman or widow was often believed to be cursed by God so every possibility was exhausted in order to bear children
Levirate marriages were widespread among Central Asian nomads. Chinese historian Sima Qian(145-87 BCE) described the practices of the Huns in his magnum opus, Records of the Grand Historian. He attested that after a man's death, one of his relatives, usually a brother, marries his widow.
The levirate custom survived in the society of Northeastern Caucasus Huns until the 7th century CE. Armenian historian Movses Kalankatuatsi states that the Savirs, one of Hunnish tribes in the area, were usually monogamous, but sometimes a married man would take his brother's widow as a polygynous wife. Ludmila Gmyrya, a Dagestani historian, asserts that the levirate survived there into "ethnographic modernity" (from the context, probably 1950s).
Kalankatuatsi describes the form of levirate marriage practised by the Huns. As women had a high social status, the widow had a choice whether to remarry or not. Her new husband might be a brother or a son (by another woman) of her first husband, so she could end up marrying her brother-in-law or stepson; the difference in age did not matter.
This type of marriage has also been practiced by many central and southern African peoples and is, to a certain degree, still in practice. In countries such as South Africa.
The marriage of Queen Gertrude to her late husband's brother is the major plot point in Shakespeare's play Hamlet. It should be noted, however, that this was not truly a levirate marriage in the strictest sense, as it was not culturally prescribed, and Gertrude already had a child (Hamlet) with her previous husband.
In the Hebrew Bible
When brothers live together, and one of them dies childless, the dead man's wife shall not be allowed to marry an outsider. Her husband's brother must cohabit with her, making her his wife, and thus performing a brother-in-law's duty to her. The first-born son whom she bears will then perpetuate the name of the dead brother, so that his name will not be obliterated from Israel.
-- Deuteronomy 25:5-6
Marriage with a brother's widow was forbidden among the Jews (Leviticus 18:16; 10:21), except for the case of yibbum. The surviving brother had the choice of halizah. Such a choice was viewed with disfavor. The brother who agreed to marry his sister-in-law would be the sole benefactor of his brother's estate instead of splitting it with the family. If the levirate union resulted in male issue, the child would be named after the deceased brother. Levirate marriage was regarded as obligatory or even permissible only when the widow had no children of either sex.
Levirate marriage is mentioned in the Bible as a standard marriage regulation among the ancient Hebrews. It is represented in many contemporary societies.
http://www.umanitoba.ca/faculties/arts/anthropology/tutor/marriage/levirate.html
This type of marriage is known as levirate marriage, from the Latin levir, “brother-in-law.” Its continuation into the NT era is demonstrated by the Sadducees’ question to Jesus about the childless woman who was married in sequence to six of her late husband’s brothers (Matt 22:23–33 = Mark 12:18–27 = Luke 20:27–40). We have seen that levirate marriage existed in Ugarit, in the Middle Assyrian (no. 33) and Hittite law codes (no. 193), and possibly in the Nuzi texts. In these texts the primary concern is with producing a (male) child to carry on the name of the deceased husband.
http://ralphriver.blogspot.com/2005/05/sexual-cleansing-and-levirate-marriage.html