What will happen to a child (18 years old) derivative immigration visa status if his principal immigration (Parents) do not maintain greencard?

An applicant who is sponsored for immigration has an obligation to make the US the country of permanent domicile. Failure to at least initially come and establish a permanent residence in the US (spending a period of time initially in the States) makes the green card statuses of the dependents insecure and subject to challenge. If your brother first enters the US and stays here for a period of approximately six months, it would make it much harder for DHS to challenge the dependents' statuses even if your brother's green card is taken away at some later point for no longer maintaining status. Due to the limitations of the Lawyers.com Forums, Alan Lee & Arthur Lee, Esqs.’ (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.