Both my grandparents died in a house fire. They were married without a contract. How will their estates be settled?

If two individuals die in the same event, and each was entitled to inherit from the other, as often occurs with spouses, each estate is settled according to the order of death (Art. 725-1 C. civ.). This order can be established by any means. If it is impossible to determine who died first, each estate will be managed without considering the inheritance rights the other co-deceased would have had (Art. 725-1 al. 2 C. civ.). In other words, it will be treated as if each of your grandparents had been widowed at the time of their death.