The "automatic" citizenship laws have changed slightly over the years, much more information is needed. Why did you not file the n600? I recommend you speak directly with an immigration attorney.
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These are the questions a qualified immigration attorney will ask you to determine whether you are able to prove derivative citizenship:
1. Were your parents married either before your brother's birth or afterwards?
2. Is your father's name on your brother's birth certificate?
3. What is your brother's date of birth?
4. Did your mother ever become a U.S. Citizen?
5. Did your U.S. Citizen parent/s live in the United States for at least 10 years or 5 years (depending upon how old your brother is), and how many of those years living in the U.S. were between the ages of 14/16-28 years? How many after 14/16?
6. Was either parent in the U.S. military?
7. Did your U.S. Citizen parent/s ever take action to claim any rights to your brother? If yes, what?
8. Was there ever a court order regarding your U.S. Citizen parent's relationship to your brother?
9. Did your U.S. Citizen parent/s ever agree to financially support your brother in writing or by court order?
10.Was the court ever involved in the relationship or support of your U.S. Citizen parent to your brother?
All these questions need to be answered and documented in order to prove a derivative citizenship case. You need to act quickly and when I say quickly, I mean you need to have an immigration attorney ready to appear on your brother's case before he signs or agrees to anything with immigration and most certainly before his next court hearing in immigration court. You also need to have an immigration attorney work with the Public Defender in case your brother is not a U.S. Citizen, to see how to lessen the immigration consequences to any criminal conviction.
I strongly suggest you follow the advice of the law professor and contact another attorney. A foreign national is not necessarily a derivative U.S. citizen just because one of his natural parents is.
It is also unusual to be charged $5,000 for a form which is available free of charge on the Internet. These are all issues which you will want to discuss with a professional.
This attorney is Board Certified, speaks Spanish and French, and has an office in South Florida. For more information, click through to my web site.