Pacto De Retro

I would like to seek for a legal advice about a parcel of land of my father in Bicol. My grandparents died without having a will and testament about this parcel of land before 1995. The older brother of my father entered into an agreement of pacto de retro sale last 1995 with one of his relatives amounting to 95k without his siblings' knowledge. My uncle created a document and let it be notarized by an attorney stating that he is the only heir to this parcel of land and so has the right to enter into an agreement of "pacto de retro". Only 5 years was allotted to regain back the said property. Now 2009, my father wants to regain back the property. But can my father still do this? Is the agreement "pacto de retro" between my uncle and his relative valid since my uncle entered into this agreement without the knowledge of his siblings who also have the right to this parcel of land?