Only natural children who have been recognized by the parents before or after the celebration of the marriage, or have been declared natural children by final judgment, may be considered legitimated by subsequent marriage.
If a natural child is recognized or judicially declared as natural, such recognition or declaration shall extend to his or her brothers or sisters of the full blood: Provided, That the consent of the latter shall be implied if they do not impugn the recognition within four years from the time of such recognition, or in case they are minors, within four years following the attainment of majority. (121a)