Family Based Immigration
A foreign national may be qualified to apply for a US green card through your parents, children, spouse, or siblings. Whether the foreign national can apply immediately or will have to wait depends upon the family relationship shared with the foreign national. Immediate relatives of US citizens include parents, spouses, and unmarried children under 21. Immediate relatives may file residency petitions without delay.
Other relatives, apart from "immediate relatives", must wait for an immigrant visa number to become available according to the following categories:
First Preference: Unmarried, adult sons and daughters of U.S. citizens. Adult means 21 years of age or older.
Second Preference: Spouses of lawful permanent residents, their unmarried children (under twenty-one), and the unmarried sons and daughters of lawful permanent residents.
Third Preference: Married sons and daughters of U.S. citizens.
Fourth Preference: Brothers and sisters of adult U.S. citizens.
Wait times will vary according to the Visa Bulletin.