My sister had a green card. when she was in USA she applied for her unmarried son above 21 years of age. for some reason she could not stay in USA. So her green card got canceled. Her son also got married. My sister no longer lives in USA. Now we got a letter saying the petition for green card is approved. Visa number yet to come. My question is, how to proceed further? what is the law to get the green card for my nephew?