Extra Judicial Partition with Confirmation of Sale and Waiver

My Father In Law, together with his mother (now deceased) and his 7 siblings executed an Extra Judicial Partition for the estate of their father.  It so happened that his late father, when he was still alive, sold to him the land that my father in law and his family built their house on some previous years ago.  They were not able to execute a deed of sale then but the family knew about the sale that took place.  In order to make the sale legal, they included it in the extra judicial settlement with confirmation of sale and waiver.  All his 7 siblings and his mother signed that said document and the estate of their late father was settled in the BIR.  When their mother died, the problem now came because 4 of his siblings will no longer honor the confirmation of sale even if they signed on the document because they said it was null and void due to the absence of a "Deed of Sale"  My father in law, brought that document to the BIR and has already settled the Capital Gains Tax from that sale and had already paid the Transfer Taxes and all other needed payments to transfer the ownership of the land to his name.  The Tax Declaration is now under his name.  He is very concerned because all their life savings were invested in that land where their business is now also located.  Is it true that his siblings can make a claim and nullify the documents they themselves signed on?  Please help!