Family Code:
Art. 178. Legitimation shall take place by a subsequent valid marriage between parents.

Art. 182. Legitimation may be impugned only by those who are prejudiced in their rights, within five years from the time their cause of action accrues.

Who could possibly challenge a first-born legitimation of spouses who just got married?

Those whose rights are prejudiced...these could be other people close to them who have interest in the legitimacy of their marriage..

Any idea who specifically these people could be?