Thank you for your question and thanks for using JustAnswer.com. While incarcerated, you still need to file a tax return if you have substantial income. If your brother was self employed and made over $400 then you would have to file a tax return. This is true even if you are incarcerated.
If you are an employee, you have to make over $9,750 before you are required to file a tax return.
Please let me know if you need further assistance with this, or if this did not sufficiently answer your question. Thanks again for using JustAnswer.com and have a happy, and safe, 2013.
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You should be aware that your question, as with most tax questions, can never really be answered completely...addressing all the permutations; that is because of the many assumptions that have to be made I have done my best to determine what I think you are asking and answering it in the most direct and understandable manner possible. If, however, after reviewing the questions, you have any uncertainties or further questions, please do not hesitate to ask.