Short answer. The person who claims that one died before the other must prove it. Under Rule 131 of the Rules of Court, when persons called to succeed each other die and the order of death is in doubt, whoever alleges a particular sequence must prove it; without proof, they are deemed to have died at the same time.
What the law says
That if there is a doubt, as between two (2) or more persons who are called to succeed each other, as to which of them died first, whoever alleges the death of one prior to the other, shall prove the same; in the absence of proof, they shall be considered to have died at the same time.
Rule 131, Section 3 — Disputable presumptions. Read the full provision →
The rule: the one alleging must prove
Rule 131, Section 3 of the Rules of Court lists the disputable presumptions, and one of them answers this exact situation. When two or more persons who are called to succeed each other — say, a parent and a child who would inherit from one another — die in circumstances where it cannot be told who died first, the rule is blunt: whoever alleges that one died before the other must prove it. Nobody is presumed to have outlived anybody. The claimant who benefits from a particular order of death carries the burden of establishing that order with actual evidence — medical findings, witness accounts, or any other proof of the sequence of events.
What happens when nobody can prove the order
If no one can carry that burden, the law supplies the answer itself: the persons are considered to have died at the same time. That legal fiction has a decisive consequence in inheritance: a person deemed to have died simultaneously with another transmits nothing to that other, because a successor must be alive at the moment the person he inherits from dies. So if a mother and her son perish together in a crash and neither's prior death can be proved, the son does not inherit from the mother and the mother does not inherit from the son. Each one's estate passes instead to his or her own other heirs, as though the other had never survived to inherit.
A different rule when succession is not involved
The same section contains a separate presumption for calamities, and it points the opposite way, which is why the two are often confused. When two persons perish in the same calamity — the rule mentions wreck, battle, or conflagration — and the case is not about them succeeding each other, survivorship is worked out from probabilities based on age and sex: for instance, if both were under fifteen, the older is deemed to have survived, and if both were over sixty, the younger. That probability table expressly does not apply to succession. For inheritance questions between the deceased, the simultaneous-death rule governs, and no presumption of survivorship arises from age or sex.
Why this matters when settling the estates
Which rule applies can move an entire inheritance from one family line to another, so it is worth being precise before agreeing to any settlement. The presumption of simultaneous death is disputable: it stands only if uncontradicted, and it can be overcome by evidence of the actual sequence — even proof that one victim survived the other by minutes can be enough to make rights pass through that person's estate. Anyone asserting such a sequence should gather the death certificates, medical or investigative reports, and witness statements early, while they can still be obtained. Where the stakes are significant, have counsel assess whether the available proof is strong enough before the estates are divided.