According to immigration law, a citizen's relatives are classified into two categories, immediate and preferential. 

Immediate family members.: These are spouses, unmarried children under 21 years of age and parents. A citizen can also petition for stepchildren and stepfathers/stepmothers, with certain special conditions, for example:

• Your spouse's child must have been under 18 years of age at the time of the marriage between you, the citizen, and the father or mother. 
• The petition must include evidence that the marriage between the citizen and the biological father or mother of the child is in good faith. 

The most important benefit of an Immediate Relative visa is that there is always a visa available for these relatives, which means that the waiting time for this category is much shorter than for the others.

Family Preferential: single children over 21 years of age is what is understood in immigration as category F1, married children of any age is what is known as category F3 and siblings of any age, single and married will be category F4.

Unlike Immediate Relatives, Preferential Relatives must wait for a visa to become available in order for their petition to be considered. This is because the number of annual visas available for these relatives is limited.

What relatives can a Permanent Resident petition for?

Permanent Residents can also “petition” certain relatives to immigrate to the United States, but these relatives, unlike the relatives of American citizens, are all in the category of Preferred Relatives.

The Preferred Relatives of a Permanent Resident are:

• Husband or wife
• Unmarried children under 21 years of age
• Unmarried children over 21 years of age

The number of visas available annually for any Preference Relative is limited. Therefore, in most cases these relatives must wait for a visa number to become available before USCIS will consider the petition.

Note that:

One of the goals of U.S. immigration policy is to keep families together.

Family Petitions do not allow American citizens or Permanent Residents to petition directly for their nieces or grandchildren, but some categories do allow Derivative Beneficiaries to immigrate who may benefit these relatives. For parents-in-law, grandparents, and uncles, there is no possibility that they can be immigrated by any petitioner.

What is a derivative beneficiary?

A derivative beneficiary refers to a relative who may be included in an immigration application, provided that he or she is directly related to the principal beneficiary of the petition.

Specific relationships: Only certain family members may be considered as derivative beneficiaries.

Dependence on immigration status: The possibility of including these relatives depends on the immigration status of the main beneficiary.

Compliance with requirements: It is also essential that the principal beneficiary meets all the requirements to immigrate to the United States.