Short answer. No. Article 757 allows reversion in favor of persons other than the donor only if they are all living at the time of the donation. If any of your cousins were not yet born when your father made the donation, the reversion clause naming them is void, though it does not nullify the donation itself.
What the law says
Reversion may be validly established in favor of only the donor for any case and circumstances, but not in favor of other persons unless they are all living at the time of the donation.
Civil Code, Article 757 — Reversion Of Donated Property. Read the full provision →
Every named beneficiary must already be alive
Article 757 imposes a strict, all-or-nothing condition for a reversion naming people other than the donor: reversion may be validly established in favor of only the donor for any case and circumstances, but not in favor of other persons unless they are all living at the time of the donation. The word 'all' matters here. If your father named several cousins as the reversionary beneficiaries, every single one of them had to already be alive at the moment the donation was made for the clause to be valid. It is not enough that most of them were born; the requirement applies to each named person.
Why an unborn beneficiary breaks the whole clause
This requirement exists because reversion to a third party gives that person a future contingent interest in the property, and the law is cautious about letting a donor tie up property indefinitely in favor of people who do not yet exist and cannot be identified with certainty. If even one of the named cousins had not yet been born when your father executed the donation, the reversion clause as a whole fails to satisfy the condition, since the article requires that they all be living at that specific moment, not merely that some eventually turn out to have been born by then.
What being void means for the reversion clause specifically
The same article addresses what happens to a reversion clause that fails this test: it becomes void, but the underlying donation is not affected by that failure. Your cousins would not have an enforceable right to receive the property upon your death based on this clause, since a void stipulation creates no rights for anyone, no matter how clearly your father intended for them to eventually benefit if you predeceased them.
What actually happens to the property if the clause is void
With the reversion-to-cousins clause void, the donation to you stands as an ordinary, complete transfer without that particular conditional feature. That generally means the property remains fully yours to pass on according to your own estate planning or the rules of succession, rather than automatically going to your cousins upon your death as your father intended. If your father is still alive, it is worth him reviewing the donation to see whether a valid reversion, limited to himself, could still achieve part of what he had in mind.