The uncle of the owner of the apartment that I wish to acquire is notaire. The seller requires that his parent write the deed of sale. Is it possible ?

No, because in accordance with Article 2 of Decree No. 71-941 of 26 November 1971 relating to documents drawn up by notaries, "Notaries may not receive documents in which their parents or allies, in a direct line, to all Degrees, and in collateral line up to the degree of uncle or nephew inclusive, are parties, or which contain some provision in their favor".