If a couple died together in an accident, who inherits from whom?

Firstly: 

When a person dies he may be inherited from by his living
heirs, so it is essential to establish whether the heir was still alive
after the person died. In the case asked about here, if it becomes clear to
us that one of the spouses died after the other, even by a short moment,
then the one who died later inherits from the first one, then the
inheritance passes to his or her heirs after that. 

But if we do not know which of the two spouses died first,
then the majority of scholars are of the view that it cannot be proven that
they inherit from one another, so neither of them inherits from the other,
because one of the conditions of inheritance is that we should establish
that the heir was still alive after the death of the deceased, and this
condition is not met in this case. 

So the husband’s estate should be divided among his heirs,
and his wife does not inherit anything from him. 

And the wife’s estate should be divided among her heirs, and
her husband does not inherit anything from her. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said,
concerning the ruling on a situation where a group of people who would
inherit from one another died in an accident. He said: When that happens,
one of the following five scenarios apply: 

1.We know exactly who died last,
so he inherits from those who died first, and not the other way round.

2.We know that they all died at
the same time, so there is no inheritance among them, because one of the
conditions of inheritance is that the heir should still be alive after the
death of the deceased, de facto or de jure, but that is not the case here.

3.We do not know how they died;
was it one after another or all at the same time?

4.We know that they died one
after another, but we do not know exactly who died after whom.

5.We know who died after whom but
then we forgot.

In the last three cases, they do not inherit from one another
according to the three imams [Abu Haneefah, Maalik and ash-Shaafa‘i]; this
is also the view favoured by al-Muwaffaq Ibn Qudaamah, al-Majd, Shaykh Taqiy
ad-Deen Ibn Taymiyah and our shaykhs ‘Abd ar-Rahmaan as-Sa‘di and ‘Abd
al-‘Azeez ibn Baaz. This is the correct opinion, because one of the
conditions of inheritance is that the heir should still be alive after the
death of the deceased, de facto or de jure, but this condition is not met
when it is not known. However the Shaafa‘is said that in the last scenario
these decisions should be delayed until they remember or agree, because
remembering is not something impossible.  

End quote from Tasheel al-Faraa’id, p. 142, 143 

Based on that, when dividing the wife’s estate: 

If the husband died after the wife, then he inherits from
her, and her estate is to be divided as follows:

The husband gets one half, the mother gets one sixth, and the
remainder goes to the brother and two sisters, with the male getting the
share of two females. 

If we do not know whether the husband died after she did,
then her estate is to be divided among the mother and siblings as follows:

The mother gets one sixth, and the remainder goes to the
brother and two sisters, with the male getting the share of two females. 

With regard to this division of the husband’s estate: 

If the wife died after he did, then she inherits from him and
his estate is to be divided as follows:

The two wives together get one eighth, the father gets one
sixth, and the children get the rest, with each male getting the share of
two females. The siblings do not get anything. 

If we do not know whether the wife died after him, then she
does not inherit from him and his estate is to be divided as mentioned
above. So the father gets one sixth, the first wife gets one eighth to
herself, and the children get the rest, with each male getting the share of
two females. 

Secondly: 

Your relative’s estate is everything that she left behind of
wealth that she owned. That includes her mahr that was given to her, whether
it was gold or cash, as well as gifts that were given to her, because they
became her property. 

With regard to the delayed portion of her mahr, this is a
debt owed by the husband, so it must be taken from his estate and added to
hers, then divided among her heirs. 

Also included in her estate is the diyah if the accident was
caused by an individual, whether it was her husband or anyone else and her
heirs asked for the diyah or it was paid by the insurance. 

With regard to the furniture and equipment in the house, they
belong to the husband unless that was part of her mahr, as is customary in
some countries, or any part of it was given as a gift by the husband to his
wife. 

Thirdly: 

The husband’s estate is whatever he left behind of wealth
that belonged to him. That includes the house that he owned. This estate is
to be divided among all his heirs. 

With regard to this issue of inheritance, reference should be
made to the sharee‘ah court so as to find out and list all those who are
entitled to inheritance and the estate, and to find out the circumstances of
death and which of the spouses died first. Because there is a difference of
scholarly opinion concerning inheritance between the spouses in such cases,
the one who should handle any cases of disagreement among the heirs is the
qaadi (judge). 

And Allah knows best.