They probably applied for humanitarian parole. Perhaps there was a disability or other urgent need. Otherwise, there is only one visa category for siblings, and it makes no distinction between married, single, under 21 or over 21.
The above is intended only as general information, and does not constitute legal advice. You must speak with an attorney to discuss your individual case.
Your brother will not be able to immigrate through a petition filed by you for at least 10 years due to limited visa availability. There is no special procedure to bring a sibling any faster aside from your parents petitioning for him upon obtaining residence.
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I agree with my colleagues. They are absolutely right.
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I agree with the other attorneys answers and would add that a sibling of a U.S. citizen falls under the Family-Based 4th Preference Category. For those from India, the current backlog for that category is about 12 1/2 years, but that wait time will change every month, depending on how many visa petitions are received for that category. For unmarried sons/daughters of permanent residents (F2B category), the wait time is about 5 years less, so it would probably be a good idea for your parents to petition your siblings after your parents become permanent residents. You can check the visa bulletin at http://travel.state.gov/content/visas/english/law-and-policy/bulletin/2014/visa-bulletin-for-march-2014.html
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