The practices of sororal polyandry and levirate marriage, according to the traditional customs of the Dulong people, dictate that in any fixed marriage group, several sisters from one family must successively marry the same husband. In the Dulong language, this is called "Anni Nan," which means marrying sisters. They believe that several sisters marrying one man can lead to harmonious relationships, strengthen familial ties, and ensure that property and labor do not leave the family. This type of marriage actually preserves the ancient custom where wives are considered internal property and cannot be transferred to external clans. It is based on exogamy within clans and thus represents a remnant of group marriage outside the clan. It occupies a significant proportion in the marital forms of the Dulong people.