Because they probably do not have an immigrant petition pending in their name and their respective parents are not LPR like yours.
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The issue is likely the pending immigration petition. The consulate may deny your B-1 application completely or they may grant it for a limited period of time. Argument for you to make is that your F3 case has 2006 date and visa bulletin shows current case date in 2003.
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The U.S. Consular Visa Section will not issue a non immigrant visa to a foreign applicant with a pending immigrant visa pending as a matter of internal policy per issued 221(g).
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Sadly even we cannot know for sure what will happen. But they are clearly worried that you will not want to return to India. 221g can take a long time. Good luck!
Dhenu Savla, Esq.
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