Are the family of the son's wife entitled to claim the deferred mahr and the gold of their deceased daughter if both spouses died in a car accident and it is not known who died first?

If the death of one spouse is known to have preceded the other, the later-deceased inherits from the pre-deceased by consensus (ijma'). If their deaths are known to have occurred simultaneously, neither inherits from the other by consensus. However, if their circumstances are unknown (i.e., it's not known who died first), then neither inherits from the other, according to the soundest opinion. This is the madhhab of a group of the Companions and the three Imams, and a derivation within the madhhab of Ahmad. Based on this, each of them inherits only from their living heirs. The wife is entitled to her deferred mahr, her debts, and whatever jewelry and furniture she owns, unless there is proof establishing her lack of ownership. The matter of inheritances is serious and must be referred to the Sharia courts for investigation and to ensure the rights of all.