Short answer. No. The testator may not make a testamentary disposition in such a way that another person is left to decide whether it is to be operative at all — that decision cannot be handed to a third party, including a sibling named in the will.
What the law says
The testator may not make a testamentary disposition in such manner that another person has to determine whether or not it is to be operative.
Civil Code, Article 787 — Validity Cannot Be Left to Another. Read the full provision →
The core rule: operativeness cannot be delegated
The statute forbids exactly the arrangement your question describes. A testator cannot write a will so that another person — your sibling, in this scenario — gets to determine whether or not a disposition is to take effect at all. Whether a gift under a will is operative has to flow from the will itself and from the law governing wills, not from a decision left in someone else's hands after the testator is gone.
Why the law draws this line
A will is meant to express the testator's own wishes about disposing of the estate. If validity itself could be left to a third party's discretion, the disposition would no longer really reflect the testator's will — it would instead depend on whatever that third party later decided, which defeats the purpose of writing a will in the first place. This provision protects the principle that a testamentary disposition's effect must trace back to the testator's own expressed intent, not to someone else's after-the-fact judgment call.
What this specifically targets
The prohibition is about the operative validity of the disposition — whether it takes effect at all — rather than about every kind of authority a will can give someone in administering or carrying out its terms. This provision would be violated by clauses purporting to give a person a veto or an on-off switch over whether a gift takes effect. It does not, by its own wording, address every other role a will might assign to a third party in the process of settling the estate, since those are separate questions this article does not itself reach.
What this means for a clause like the one you describe
If your father's will actually contained language purporting to let your sibling decide whether your inheritance takes effect, that specific clause runs against this article and would not be given effect as written. The disposition in your favor should instead be evaluated on its own terms — under the will's actual wording and the general rules on testamentary succession — rather than being left contingent on whatever your sibling might later decide.