Short answer. It is valid under Article 869 of the Civil Code, but with important limits borrowed from the fideicommissary substitution rules. When a testator gives a usufruct to one person and then, successively, to another, the arrangement must comply with Article 863: both usufructuaries must be living at the testator's death, and the succession cannot extend beyond one degree.
What the law says
If he gives the usufruct to various persons, not simultaneously, but successively, the provisions of article 863 shall apply.
Civil Code, Article 869 — Successive Usufruct. Read the full provision →
Article 869 expressly allows successive usufructs
Article 869 of the Civil Code validates two separate testamentary arrangements in a single provision. First, it allows a testator to split ownership and enjoyment — leaving the naked ownership to one person and the usufruct to another. Second, it addresses successive usufructs: the testator can give the usufruct first to one person and then, after that person's usufruct ends, to another. If he gives the usufruct to various persons, not simultaneously, but successively, the provisions of article 863 shall apply. The arrangement your grandmother created is the second type, and it is valid — subject to the conditions Article 863 imposes.
The fideicommissary rules that apply
Article 863, which governs fideicommissary substitution, sets two conditions that a successive usufruct must satisfy. First, the one-degree limit: the succession cannot go beyond one degree from the first usufructuary. In your situation, your father receives the usufruct first, and you are next. You are his child — one degree — so the requirement is satisfied. Second, the survival requirement: both the first usufructuary (your father) and the second (you) must be living at the time of the testator's death — that is, at the time your grandmother died. If you were not yet born or had already died when your grandmother passed, the successive grant to you would be invalid.
What the first usufructuary's obligations are
Borrowing from the fideicommissary framework, Article 863 entrusts the first usufructuary — your father — with the obligation to preserve and to transmit the usufruct to the second. This mirrors the duty of a fiduciary heir: enjoy the benefits of the usufruct during your lifetime, but preserve the right so that it can pass to the next person. Your father cannot act in ways that would permanently destroy the usufruct or strip it of substance before it reaches you. Once your father's usufruct ends — typically when he dies — the usufruct passes to you.
Practical implications for your situation
If both you and your father were alive when your grandmother died, the arrangement is valid, and you can expect to succeed to the usufruct when your father's entitlement ends. The property's naked owner — whoever your grandmother left the ownership to — holds ownership throughout but cannot enjoy the fruits while the usufruct is in force. Keep a copy of your grandmother's will and any probate order establishing the arrangement. When the time comes for you to assert your successive usufruct, documentation of both your grandmother's testamentary disposition and your father's prior enjoyment will matter. A lawyer can advise on how to formally record your right and what it entitles you to under the Civil Code's usufruct rules.