These are the three categories in which the approval of the relative petition ("Form I-130") has no waiting time in which to file for a green card: a. Spouse of US Citizen; b. Minor Child (under 21) of US Citizen; and c. Parent of Adult US Citizen (over 21). That's it. Every other family-based category has a wait time after the petition is approved. Of course, you must show that the relationship is valid. This is accomplished by attaching birth certificates, citizenship papers, marriage certificates, etc. In some cases, when Immigration is not satisfied by presentation of fundamental documents, there might be a need to show sworn statements from family and friends, and in the most extreme circumstances, DNA testing. The seemingly simple process is deceptively easy-looking. You have one chance to do it right, Contact an experienced, well-regarded immigration attorney before filing.
This category is for the Unmarried Sons and Daughters of US Citizens. These are the single children, over 21, of persons either born in the US or those who naturalized. At the time of this writing, once their I-130s are filed, these folks are waiting five years or more to reach the point when they will be eligible to apply for green cards (assuming approval of petition). Not unlike Immediate Relatives, First Preference requires presentation of papers such as birth certificates and citizenship papers, again with rarer cases needing DNA testing.
Second Preference A
This section relates to Spouses and Minor Children (under 21) of green card holders (lawful permanent residents). 2As are not waiting that long these days, after I-130s are filed, before filing for the green card. That wait time, however, may be extended depending on how many other people file in any given month, as there are quotas in all preference categories (unlike Immediate Relatives). Lately we have seen the waits to be anywhere from a few months to several years. As noted above, marriage-based immigration typically requires an interview to verify the validity of the relationship. Also, applicants who already live together must be ready to show evidence of joint residence (a "shared life").
Second Preference B
2Bs are Unmarried Sons and Daughters (over 21) of green card holders. There is no category for married children, age 21 and older, of lawful permanent residents. Second Preference B historically has been backed up quite a bit; it is not unusual for 2Bs to wait seven or eight years to file for green cards. Birth certificates are, again, required in these cases. When birth certificates are unavailable, there are provisions for affidavits and other secondary evidence but these proofs are limited to nations in which birth records are not readily available. Between the long waits and other complications along the way, it is important to call a veteran immigration attorney with an excellent reputation, before filing I-130s.
This category is for the Married Sons and Daughters of US Citizens. These are the married children, over 21, of persons either born in the US or those who naturalized. At the time of this writing, once their I-130s are filed, these folks are waiting seven years or more to reach the point when they will be eligible to apply for green cards (assuming approval of petition). However, Mexico, which has its own category, along with several other countries, is backed up 18 years in both 2B and 3rd right now, due to the oversubscribed quotas. Not unlike Immediate Relatives, Third Preference requires presentation of papers such as birth certificates and citizenship papers, again with rarer cases needing DNA testing.
This section is for Brothers and Sisters of Adult US Citizens. Fourth Preference wait times have historically been 8 to 12 years or more, before reaching the point to be eligible to apply for green cards (assuming approval of petition).
Spouses and minor children of beneficiaries
In some of the above categories, husbands or wives and children under age 21 may accompany the family member when the green card is approved. For example, this rule is applicable in the cases of minor children of First Preference, as well as spouses and minor children of Third and Fourth Preference.
There is no category for married children, age 21 and older, of lawful permanent residents, as noted above. Nor all there cases for uncles, aunts, cousins, in-laws and grandparents, although it never hurts to carefully analyze the family tree to see if one can find a suitable relative petition that might not appear readily available. Also, change in status will change the preference category. Divorce, death, and marriage can obviously affect the status of an I-130. I cannot reiterate enough how much you should think about speaking with a lawyer who is very well versed and well-known in the field of immigration, before trying to file family petitions on your own.
This guide is in no way meant to be exhaustive. It is an introduction to family immigration law. There are other topics, such as widows, fiancees, and adoption, which may be covered in another guide.