A man and his wife, son and daughter died in an accident; how should their estates be divided?
Question

                My sister and her husband, son and daughter died in an accident.

The wife and husband died at the same time.

Then the daughter died four days later, and the son died four days after his sister.

The wife had a mother, brothers and sisters through the father and mother, brothers and sisters through the father only, and brothers and sisters through the mother only.

The husband had brothers and sisters through the father and mother.

How should the estate be divided among them?.
Praise be to Allah.
    If a number of 
    people die in an accident or in a collapsed building or fire, and they could 
    inherit from one another because of marriage or blood ties, the one who dies 
    later inherits from the one who dies earlier. If they die at the same time, 
    they do not inherit from one another. 

    Shaykh Ibn ‘Abbaas 
    (may Allah have mercy on him) said: If two or more people who could inherit 
    from one another die in a collapsed building, by drowning, in a fire or 
    plague and the like, then there are five scenarios, one of which is that if 
    one of the two who could inherit from one another dies after the other, even 
    if only by a moment, then the one who dies later inherits (from the one who 
    dies first), according to scholarly consensus.

    The second 
    scenario is that if they die at the same instant, there is no inheritance, 
    according to scholarly consensus.

    End quote from 
    al-Fawaa’id al-Jaliyyah fi’l-Mabaahith al-Fardiyyah. 

    Based on that, the 
    spouses do not inherit from one another in this case, because they died at 
    the same moment. 

    Then we should 
    look at the case of each deceased person: who among his heirs was still 
    alive after his death? 

    The heirs of the 
    wife are only her daughter, son and mother. Her siblings, whether through 
    both parents or through the father or mother only, do not get anything, 
    because they are prevented from inheriting by the presence of the son. 

    Based on that, the 
    mother gets one-sixth and the rest goes to the son and daughter, with the 
    male getting a share equal to that of two females. 

    The son and 
    daughter inherit from the husband, with the male getting a share equal to 
    that of two females. His siblings do not get anything because they are 
    prevented from inheriting by the presence of the son. 

    The brother and 
    grandmother (mother’s mother) inherit from the daughter, so the grandmother 
    gets one-sixth, and the rest goes to the brother. 

    The paternal 
    uncles (brothers of the father) and the grandmother (mother’s mother) 
    inherit from the son. The grandmother gets one-sixth and the rest goes to 
    the paternal uncles. The paternal aunts get nothing. 

    The estate should 
    be divided in this order. Attention must be paid to giving the son and 
    daughter their shares from their father’s and mother’s estates, in addition 
    to whatever wealth of their own they had. And attention must be paid to the 
    fact that the grandmother (mother’s mother) will inherit from the mother 
    (the wife) and from the son and daughter. 

    And Allah knows 
    best.