Is Your Family Member an "Immediate Relative?"
“Immediate relatives” of a U.S. citizen are defined as a spouse, unmarried children under the age of 21, and parents. Immediate relatives always have a visa number immediately available.
Family Preference Category
If the family member of the U.S. citizen is not an immediate relative, then the U.S. citizen may still be able to sponsor them via what is called a “family preference category.” Eligible relatives include:
Congress has limited the number of relatives who may immigrate under these categories each year so there is usually a waiting period before an immigrant visa number becomes available.
Get a Green Card While Inside the United States
If you are currently in the United States and are one of the specified categories of relatives of a U.S. citizen in a preference category, you may be able to become a permanent resident in two steps.
Get a Green Card While Outside the United States
If you are currently outside the United States and are one of the specified categories of relatives of a U.S. citizen in a preference category, you can become a permanent resident through consular processing. Consular processing is when we work with the U.S. Department of State to issue a visa on an approved Form I-130 petition when a visa is available. In this process the Department of State will issue you a visa. If approved, you may then travel on the visa and will officially become a permanent resident when admitted at a U.S. port of entry.
Things to keep in mind:
Source: United States Citizenship and Immigration Services (USCIS)