The U.S. immigration system permits immigration based on specific familial relationships. Those are broken down into two categories: immediate relatives and family preference relatives. These categories define the type of relationship that the immigrant has with the U.S. sponsor, and by and large, the priority that the immigrant will receive in obtaining a green card.
Immediate relatives relate directly to close family members of United States citizens. These are parents, spouses, and children (underage of 21) of United States citizens. There is no limit on the number of immediate relatives that can obtain immigrant visas in a year.
The family preference categories include unmarried sons and daughters of U.S. citizens, and their minor children; spouses, minor children, and unmarried sons and daughters (age 21 and over) of Lawful Permanent Residents; married sons and daughters of U.S. citizens, and their spouses and children; and brothers and sisters of U.S. citizens, and their spouses and minor children, provided the U.S. citizens are at least 21 years of age.
Your position in the chronological quota system is determined by your priority date; the date on which your Form l-130 was properly filed with USCIS.
In certain situations, an applicant may benefit from the charging of their visa to their spouses or parent’s country of birth rather than their own. This is known as cross-chargeability.
In practice, cross-chargeability is used where the preference quota category is backlogged for one spouse’s country of chargeability but is current for the other spouse’s country of chargeability. The principal applicant can charge their visa to the derivative spouse’s country, and the derivative spouse may cross-charge to the principal’s country.
Children are categorized as being unmarried individuals under 21. Consequently, it’s pretty common that a person, who was a child when their immigration petition was filed, can no longer be classified as a child when their visa would become current.
E.g., it can take 15 years before an F-4 sibling visa becomes current and if you are the child of the principal beneficiary i.e., your uncle filed for your mother when you were 15 years old, by the time you are eligible for the visa, you will be 30 years old and will be considered to have “aged-out”.
Yes. It depends on a few factors and relies on a Child Status Protection Act (“CSPA”) formula. The USCIS has plenty of information about that over on their website http://www.uscis.gov.