All family-based immigration begins with a petition, either the I-130 or the I-129F (for fiances). These petitions prove your family members eligibility for a visa. For these petitions, generally you need to show the petitioner’s status in the United States, certificates of family relations, and evidence that the relationship is real (depending on the case).
Once the family petition is approved, the beneficiary can either apply abroad for an immigrant visa or apply in the United States for adjustment of status, if a visa is immediately available.
For parents, spouses, fiances, and minor children of US citizens, there is always a visa available, meaning the beneficiary may continue with the process as soon as the family petition is approved.
In the other categories, there are a limited number of visas each year, so the process may be delayed, sometimes by many years. For example, it may take 12-14 years for a visa to open become available for a brother or sister of a US citizen. The Visa Bulletin provides the dates of visa availability.