US citizens can also sponsor adult children, married children, siblings (brothers and sisters) for family based immigration.
A married child can bring his/her spouse and eligible child/children.
If a sibling is married his/her spouse and eligible child/children can also immigrate.
A child is eligible if unmarried and under 21.
Permanent Residents can only sponsor spouses and unmarried children.
Each preference category is allocated a number of the available visas.
F-1 unmarried children over 21 of US citizens : 23,400
F-2A spouses and unmarried children under 21 of US residents: 87,934
F-2B unmarried children over 21 of US residents: 26,266*
F-3 married children of US citizens with eligible family members: 23,400
F-4 brothers/sisters of US citizens with eligible family member: 65,000
* More visas can be issued for F-2 preferences.
For some preferences the waiting times are long to very long. Especially for family members waiting to immigrate from Mexico, China, India, the Philippines. Not only because of the large numbers of family-sponsored immigrants-in-waiting; these countries are also much disadvantaged by the per country limit.
Waiting times can be found in the Visa Bulletin published monthly by the US Department of State.