Does an heir’s share vest the moment the person dies?

Agreed. Yes — at the moment of death, automatically. The Kuwaiti Mawsu’a puts the point negatively first and then positively. Negatively: “acceptance of the inheritance is not a condition of the estate passing to the heir, nor is his taking time to consider before accepting it”. Positively: “rather it falls to him compulsorily, by the ruling of the sacred law, without any acceptance from him”. It then states the point of transfer twice more in agreement language. Once at the head of the chapter on the estate’s ownership: ownership of the estate passes compulsorily to the heirs, and “the jurists have agreed that the transfer of the estate from the deceased to the heir takes place after the death of the deceased — actual, constructive or presumed”. And once on timing, for the ordinary case of a person who dies with no illness before it — a heart attack, an accident: the heir succeeds his deceased “at the very moment of death, and with no disagreement worth counting among the jurists”. That is why the Qur’an describes the shares as being in “what the parents and near relatives leave”. Two consequences follow and both matter in practice. First, an heir cannot be treated as having lost his share by delay, absence, or not having signed anything — and an heir who survives the deceased and then dies before the family divides the estate does not lose it either; it passes on to his own heirs. Second, and this is the point the site once got wrong: the question of what belongs to the estate is a question about ownership at the moment of death. Whether an asset had to be probated in Ontario, or passed outside probate by survivorship or a beneficiary designation, is a fact about Canadian administration. It is not the test any fiqh source applies, and no school treats it as one. We checked this point; the schools do not differ on it.

Shared evidence: al-Mawsu’a al-Fiqhiyya al-Kuwaytiyya, “Tarikah” sec. 13 — “acceptance of the inheritance is not a condition... rather it falls to him compulsorily, by the ruling of the sacred law, without any acceptance from him” · The same entry, sec. 7–8 — ownership of the estate passes compulsorily to the heirs, and “the jurists have agreed that the transfer... takes place after the death of the deceased, actual, constructive or presumed” · The same entry, sec. 15 — for a sudden death the heir succeeds “at the very moment of death, and with no disagreement worth counting among the jurists” · Surah an-Nisa 4:7 — a share in “what the parents and near relatives leave”.

Not covered by this finding: the case of an estate that carries debt. The agreement quoted above is about the moment and the automatic character of the transfer. Where debts attach to the estate the schools genuinely differ over whether the assets themselves pass to the heirs at once with the debt attaching to them, or stay in the deceased’s ownership until the debt is paid — and what turns on it is who takes the rent, profit and appreciation earned in between. That difference is set out, with each school separately sourced, in the register’s estate-valuation-date entry, and nothing here should be read as answering it. — not yet researched school by school. Treat that as open and ask a scholar of your school.