My sister and her exhusband are unfit to be with their children.
1 is 10 and the oldest is 15 yrs old. Many times the kids have told me that they don't want to be with either of their parents.
I live in New York city, please advise.

Can you describe why you think they are unfit? Just because their kids down't want to live with them isn't going to be good enough to get them out.

First you need to document why they are unfit. Then you need to talk to DCS and ask them to investigate. If they determine the parents are unfit, then you can petition the court to become their guardians.