回复：Thanks 823611, one more question

My sister may not stay in US either, just her kid stays  US to study at college.

If both my sister and her hu*****and give up green cards,  will her child still apply for green card for them again when his he becomes US citizen ?

-Yes if he is 21 years old or more and has the financial ability. Bythe way, I would suggest that the father also enters US to get the green card and then, give up the green card later with the mother, thus it is very easy for them to apply for the B2 visa to visit the child any time in the future.