Problems on Extra-Judicial Partition

Good day admin. My parents has inherited a few parcels of land from their parents who already passed. The extra-judicial partition that they have is for the estate of their father who passed last 1994. In the said settlement is an inclusion of waiver of claim and confirmation of sale on one of the lots in favor of my father. This transaction was made by my father and grandfather when he was still alive because he needed the money. It was confirmed by my grandmother that indeed the said transaction took place. My father, in his own good intention, did not want to take the whole lot to be his, but only the portion of the lot equivalent to the amount of cash borrowed. Now they executed the extra judicial partition wherein all of the heirs (all his children) and his wife signed that said document. It was duly submitted to the BIR. Last January 2014, my grandmother passed away and they started preparing for the settlement of her estate. Only now, my father's siblings questioned the waiver and confirmation of sale and they want to dispute it. My father is already old and going through this ordeal is taking a toll on his health. His siblings are threatening to sue him for whatever case that they don't even want to tell us. Can you please give us an advice as to how are we going to go with this problem?

Are the any more evidience the trade took place?
As:
Deed of Sale or Reciept signed by grandfather?
Who has paid the property tax?
I suppouse no transfer of title or was it? Any notes about the trade at the mother title?
Was BIR and/or the local property tax office told about the trade?

Your father taking a loan is a sign, but not a proof by itself, because the others can claim the loan was to something else.