My partner appointed me as the sole legatee of his property upon his death. Can his brothers and sisters still take over the property he received from their parents, by inheritance?

No. The universal legacy makes it possible to set aside the right of return of the brothers and sisters, including if it is made for the benefit of the Civil Union partner of the testator (art. 757-3 C. civ. et Cass. Civ. 1e, 28 Jan. 2015, n°14-20.587).