Hello!
I am from Rumania. A week before we understood that my father-in-law's cousin has died at NY. He was an american citizen with no children and close relatives. He made a will giving all to a NGO in Rumania. He is in dept of his uncle (my father-in-law's father) who helped him.
What we can do?

Any valid debts need to be paid before the proceeds of the estate can be paid to the inheritors. You need to file a claim against the estate.

Bur having said that... people can leave their money to whoever they want. Especially if they aren't married and have no kids. So while you can file a debt against the estate, you'd have to prove that he was of unsound mind or unduly influenced when he made the will and left the money to the charity. If you can't do that, the money will go to the charity.

To expand on what I said, I don't see any grounds to contest the will. A cousin or uncle doesn't need to be named in the will. So your best bet is trying to get the debts paid from the estate.