Does the mother inherit from her daughter who died with her in a car accident, given that it is unknown who died first, and that the daughter has a husband and two daughters?

There is no inheritance between a mother and her daughter if it is unknown which of them died first, because one of the conditions for inheritance is that the heir must be alive after the deceased. Thus, the daughter's estate is distributed among her heirs, excluding the mother, and similarly for the mother's estate.