Parallel cousin marriage is a less prevalent institution, but does occur
in some societies.
In this situation the children of two same sex siblings marry.
Patrilateral parallel cousin marriage, the marriage of brothers' children is the standard pattern. When parallel cousin marriage is practiced in societies organized into unilineal descent groups, it has the effect of arranging marriages within the lineage and results in lineage endogamy.
The Bible offers an interesting case of lineage endogamy among the generations of the Hebrew patriarchs (See Ancient Hebrew Social Organization). It also contains a detailed consideration of parallel cousin marriage at the very end of the Book of Numbers. At this point Moses has apportioned some TransJordanian land to three of the tribes of Israel prior to the settlement of Canaan. Two leaders of the "house" (i.e. lineage) of Manasseh approach him with a problem that will result from an inheritance rule (detailed earlier in Chapter 27) that specifies that a woman inherits her father's property if he has no sons. In this event the property will be passed on to the daughter's children and into her husband's lineage. Moses solves the problem by advocating a new regulation: a woman is to marry her father's brother's son (patrilateral parallel cousin marriage) so that her husband and children will belong to the same lineage as her father and the group property will remain intact.
"Even as the LORD commanded Moses, so did the daughters of Zelophehad: For Mahlal, Tirzah, and Hoglah, and Milcah, and Noah, the daughters of Zelophehad, were married unto their father's brothers' sons: And they were married into the families of the sons of Manasseh the son of Joseph, and their inheritance remained in the tribe of the family of their father." Numbers 36.