HI !
As I undersatnd that your grandparents are desperate to see thier grandchildrens ( which is obvious ) you invite them here on visitor visa . Once they are here you can file their immigration from Buffalo which is much faster and easier ( One of my known person did it ) ..
Well once your parents are here you will be listening a english saying ;
If I knew granchildren will be so much fun I wud hv them first ...LOL
Wish you best in all your endeavours ..
Cheers
Ashwani

P.S: You might consult a immigration expert on the above situation b'cos I do not know how your sister will be eligible under that criteria ..

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