Short answer. Yes, it can be valid. Article 843 of the Civil Code allows a testator to designate an heir by description instead of by name, as long as there is no doubt about who is meant. A description like 'my youngest daughter who lives with me' that points to one identifiable person is sufficient.
What the law says
Even though the testator may have omitted the name of the heir, should he designate him in such manner that there can be no doubt as to who has been instituted, the institution shall be valid.
Civil Code, Article 843 — Naming the Heir. Read the full provision →
A name is the default, not a strict requirement
Article 843 of the Civil Code sets out the general rule first: the testator should designate the heir by name and surname, and where two people share the same name, should add a distinguishing detail. But the article then immediately supplies a safety valve. Even if the testator omitted the heir's name, the institution is still valid as long as the description leaves no room for doubt about who was meant. Form gives way to clear intention: what matters is that the person can be identified with certainty.
When a description is certain enough
A phrase like 'my youngest daughter who lives with me' works when exactly one person fits that description. If your grandfather had only one youngest daughter and she was in fact residing with him at the time he wrote the will, the identification is unambiguous. The description is not merely a label — it is a factual reference that anchors the designation to a real, identifiable person. Courts look to the facts existing at the time the will was made to determine whether the description points unmistakably to one individual.
When description creates doubt instead of certainty
The rule flips if the description is ambiguous. If the testator had two daughters who both lived with him at different times, or if the description fits more than one person, the institution may be challenged as insufficiently certain. Ambiguity does not automatically void the institution — courts try to resolve uncertainty by looking at surrounding circumstances, other parts of the will, and extrinsic evidence — but the more people a description might cover, the weaker the institution becomes. No doubt must remain after all the evidence is considered.
Practical implications during probate
When a will identifies an heir by description, the probate court will need to determine as a factual matter that the description points to the right person. Evidence of who your grandfather's daughters were, which one lived with him, and the circumstances at the time of writing will all be relevant. If other heirs dispute the identity of the person meant, this becomes a contested issue. Getting legal assistance early in the probate process ensures that the claim of the identified heir is properly established from the start.