woma selling land, 4 sons are ready to countersign but one is NRI

I am trying to buy a plot/ land, which is registered in the name of say "X(Woman)" she got property from her husband (who bought this land on earned money) after passed away, she is having four sons at that time and we don't know whether their sons voluntarily registered the land on her name or not, but now they are married and having children also.. she wants to sell the land and his 3 sons are ready to countersign, Fourth one is also ready, but he is not in India to sign at the time of registration. She is saying orally that no problem will arise in future either legal or personal. Is it required to have the counters signatures of her sons, if So & if he cannot sign what are the possible options I have to buy the land? Thanks & Regards.