U.S citizens can bring their immediate relatives who are outside to the U.S into the country by applying for Petition for Alien Relative, Form I-130, for spouses. Parents and children of U.S citizens are eligible for the immediate relatives preference category along with the spouse. Siblings of U.S citizens however are eligible for the family preference category.
When a U.S citizen wants to bring over their immediate family members from another country, care should be taken to file individual application forms for their spouse and minor children. It is a common misconception that minor children and step-children of U.S citizens are included when filing Form I-130, Petition for Alien Relative, for spouses. But the USCIS processes individual application forms for the spouses and children of U.S citizens. So, care must be taken to apply separately for each individual, including minor children.
Siblings of U.S citizens, who are minors, are not included in the application filed for the parents. Separate immigrant applications have to be filed for the parents of the U.S citizen and the minor siblings, for obtaining a Green card for the family members.
However, under the family preference category and the employment category petitioners can include minor children in their applications. This enables the minor children of the immigrants to obtain immigrant visas as derivative beneficiaries. This is applicable only for minor children of immigrant visa holders and not for minor children of U.S citizens.