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.
Reproduced from Islam QA