1. If the grandfather alone was its owner and he has not left any Will to make your grand mother its sole owner then on his death your grand father and all the children become its co sharer in undivided equal share.
2. In that event you grand mother alone can not sell the whole proeprty and to do so he requires signature of her children also.
3. If her children are so willing they can make a gift deed giving their respective share to their mother so your grand mother becomes its exclusive owner and then she can sell it and enjoys the sale proceeds.
So act accordingly.