Your parents may file for their son so long as he is not married and remains unmarried until an immigrant visa becomes available or they naturalize. A lawful permanent resident can only petition for a spouse, child under 21 or unmarried son/daughter. Visa availability is often faster for petitions filed by parents over siblings. Whether this is true for your case will depend upon when your sister applied. You can read more about family immigration at http://myattorneyusa.com/family-immigration.
Answered on May 17th, 2016 at 3:46 AM