I advise you to contact a competent immigration attorney to evaluate the specific details of your matter. In general, to claim US citizenship because of the US citizen parent, a foreign adopted child must reside legally in the U.S. in the legal and physical custody of the U.S. citizen parent AND meet following conditions before 18th birthday:
(1) The US citizen parent adopted the child before his or her 16th birthday (or, in some cases, 18th birthday) and had legal custody of the child and resided with the child for at least two years; OR
(2) The child was admitted to the United States as an orphan (IR-3) or Convention adoptee (IH-3) whose adoption by his or her U.S. citizen parent(s) was fully completed abroad; OR
(3)The child was admitted to the United States as an orphan (IR-4) or Convention adoptee (IH-4) who was coming to the United States to be adopted and the child's adoptive parent(s) completed the adoption before his or her 18th birthday.
As stated above, please contact a competent immigration attorney for your specific case. Most of us provide free 30 minute consultation by Skype. Any answer provided here is not intended to be legal advice. No attorney client relationship is intended or formed.
You and your brother would be best advised to consult an experienced immigration attorney. More information is needed to assess the matter such as how you entered the country, where was the adoption completed, did your ad I price parents ever petition for you, etc. Even if you cannot claim citizenship through your parents, you can seek naturalization after you have been a permanent resident for the requisite number of years.
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Schedule a consultation with an Atlanta immigration attorney at once to determine if you are already a U.S. citizen and if not, how best to bet on the path to obtain citizenship since it appears you might be eligible at least down the road IF you follow the correct process.
This advice does not form an attorney-client relationship and is merely informative. It should not by itself be relied upon to address a legal concern.
It is possible that you could, though a lot more information would be needed to determine whether that would be an option. If you're not eligible through you parents, there may be other options for you and/or your brother to ultimately get your citizenship. Determining which options are available, and which would be best, would require a much more in depth analysis of your situation than could be provided on this website. You should consider setting up a consultation with an immigration attorney and going through all of the specifics of your and your brother's situation so that a more thorough assessment can be done.