Preference relatives are certain family members of a U.S. citizen who are seeking to immigrate to the United States based on their relationship to the U.S. citizen. There are yearly limitations on family preference immigrants and the relatives in this category must often wait several years before an immigrant visa becomes available. The available visas are issued in the order in which the petitions were filed (the priority date).
The family preference categories include unmarried sons and daughters of U.S. citizens, and their minor children; spouses, minor children, and unmarried sons and daughters (age 21 and over) of Lawful Permanent Residents; married sons and daughters of U.S. citizens, and their spouses and children; and brothers and sisters of U.S. citizens, and their spouses and minor children, provided the U.S. citizens are at least 21 years of age.
Bureau of Consular Affairs, Family Based Immigrants, travel.state.gov/visa/immigrants/types/types_1306.html
USICS, Family of U.S. Citzens, www.uscis.gov/portal/site/uscis/menuitem.eb1d4c2a3e5b9ac89243c6a7543f6d1a/