Will the deceased son inherit from his father if it is not known who died first in a car accident? And is it permissible for the brothers to pay off their deceased brother's debt from the value he paid for his apartment and then take possession of the apartment?

If two heirs die, and it is not known which of them died first, there is no inheritance between them. Upon a person's death, the estate begins with their preparation and shrouding, then their debts are paid, then their وصايا (bequests) are executed from one-third of the estate, and then the remainder is distributed to the heirs, as Allah Almighty says: (from after any bequest which he may have bequeathed or a debt). Accordingly, the deceased's debts must be paid before distributing their estate. If the apartment was owned by the deceased and they had a debt from its price and other debts, then the apartment must be appraised at a fair price. Their siblings can buy it, and the deceased's debt is paid from its price, with the remainder going to their heirs. It is not permissible for the siblings to bear the debt and for the apartment to be theirs, with only a small amount remaining for the heirs, unless the current value of the apartment does not exceed the total of the debt and the amount remaining for the heirs after a fair appraisal.