From USCIS website:
Eligibility
In order for a relative to sponsor you to immigrate to the United States, they must meet the following criteria:
•They must be a citizen or lawful permanent resident of the U.S. and be able to provide documentation providing that status.
The relatives which may be sponsored as an immigrant vary depending on whether the sponsor is a U.S. Citizen or a lawful permanent resident.
•If the sponsor is a U.S. Citizen, they may petition for the following foreign national relatives to immigrate to the U.S:
•Husband or wife
•Unmarried child under 21 years of age
•Unmarried son or daughter over 21•Married son or daughter of any age
•Brother or sister, if the sponsor is at least 21 years old, or
•Parent, if the sponsor is at least 21 years old.
•If the sponsor is a lawful permanent resident, they may petition for the following foreign national relatives to immigrate to the U.S.:
•Husband or wife, or
•Unmarried son or daughter of any age.In any case, the sponsor must be able to provide proof of the relationship.
Preference Categories
If you wish to immigrate as a relative of a U.S. Citizen or lawful permanent resident, you must obtain an immigrant visa number based on the preference category in which you fall.
People who want to become immigrants are classified into categories based on a preference system. The immediate relatives of U.S. citizens, which includes parents, spouses and unmarried children under the age of 21, do not have to wait for an immigrant visa number to become available once the visa petition filed for them is approved by USCIS. An immigrant visa number will become immediately available. The relatives in the remaining categories must wait for an immigrant visa number to become available according to the following preferences:
•First preference: Unmarried, adult sons and daughters of U.S. citizens. Adult means 21 years of age or older.
•Second Preference: Spouses of lawful permanent residents, their unmarried children (under twenty-one), and the unmarried sons and daughters of lawful permanent residents.
•Third Preference: Married sons and daughters of U.S. Citizens.
•Fourth Preference: Brothers and sisters of adult U.S. Citizens.
Once USCIS receives your visa petition (Form I-130, Petition for Alien Relative), it will be approved or denied. USCIS notifies the person who filed the visa petition if the petition was approved. If you are living outside the United States or choose to obtain your visa outside the U.S., USCIS will then send the approved visa petition to the Department of State's National Visa Center, where it will remain until an immigrant visa number is available. The Center will notify the foreign national when the visa petition is received and again when an immigrant visa number is available. You do not need to contact the National Visa Center, unless you change your address or there is a change in your personal situation, or that of your sponsor, that may affect eligibility for an immigrant visa, such as reaching age 21, marriage, divorce, or death of a spouse. If you are living within the United States and choose to adjust status in the United States, USCIS will adjudicate your Form I-485. You (and your petitioning relative) may be required to appear for an interview at a local USCIS field office. You should also advise USCIS of change of address or any change in your personal situation, or that or your sponsor, that may affect your eligibility to adjust status.
More info can be found at
http://www.uscis.gov/portal/site/us...nnel=4f719c7755cb9010VgnVCM10000045f3d6a1RCRD
