Short answer. Yes. The legitimation of children who died before the celebration of the marriage benefits their descendants. Death does not remove your brother from the operation of the rule; his legitimation still takes place, and its advantages pass to his children.
What the law says
The legitimation of children who died before the celebration of the marriage shall benefit their descendants.
Family Code, Article 181 — Legitimation Benefits the Descendants of a Deceased Child. Read the full provision →
A dead child is still legitimated
The instinct is that legitimation must be for the living, since a person who has died has no status left to improve. The article rejects that. It speaks of the legitimation of children who died before the celebration of the marriage as something that happens, and then directs where its advantage goes. The reason is that status is not only personal — it determines what passes through a person to those who come after — and to let death cut the chain would punish grandchildren for the timing of a wedding they had nothing to do with.
What the descendants actually gain
The benefit is derivative rather than a status of their own. Because your brother is treated as legitimated, his children stand in the position that legitimacy gives him when a right passes through him — most concretely in the succession to your parents, where the share he would have taken as a legitimate child is what his descendants may claim through him. Coupled with the rule that the effects of legitimation retroact to the time of the child's birth, this can reach back to matters that were settled long before anybody thought about it.
The underlying legitimation still has to hold
The article distributes a benefit; it does not create one where legitimation could not occur. Whether your brother was legitimated depends on the ordinary requirements — the parents' subsequent valid marriage, and their situation at the time he was conceived. If those are not satisfied, this article has nothing to operate on, and the descendants take, if at all, in the position of descendants of an illegitimate child. So the first question is not about your brother's death but about your parents' circumstances years before it.
Assemble three generations of documents
Claims of this kind live on civil registry records. You will want your parents' marriage certificate, your brother's birth certificate, his death certificate, and the birth certificates of his children showing his paternity. Filiation of the grandchildren is often where the difficulty sits, particularly where he himself was not married. Note too that legitimation may be impugned by those prejudiced in their rights within five years from the accrual of their cause of action, so a family settlement drawn without regard to these children is worth raising with a lawyer promptly.
Related provisions
- Family Code, Article 181 — Legitimation Benefits the Descendants of a Deceased Child
- Family Code, Article 180 — Legitimation Retroacts to Birth
- Family Code, Article 182 — Who May Impugn a Legitimation