All perfect praise be to Allah the Lord of the Worlds. May Allah`s peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
The relatives killed in an accident don`t inherit each other if it isn`t known whose death took place earlier. Muslim scholars have discussed this issue under "Chapter on Inheritance of Persons Killed by Drowning and Building Collapse."
Al-Shirbin (May Allah have mercy on him) stated: "If two relatives were killed together by drowning, fire, collapse of building, in a foreign country or it wasn`t known whose death took place earlier, then none of them inherits the other. This is because it is conditioned that one of them is alive to inherit the other and, here, this condition isn`t fulfilled. Moreover, the estate of each one of them is transferred to the living heirs because, according to Islamic Law, the living inherit the dead and, here, it isn`t known whose death took place earlier, so none of them inherits the other." {Moghni al-Mohtaj, vol.4/pp.47}.
Based on the above, the estate of those killed in the accident must be divided between their heirs: son and daughter. From their father, the male gets twice the share of the female. From their mother, the male gets twice the share of the female. From their brother, the male gets twice the share of the female. This is attested to in the verses: " God (thus) directs you as regards your Children’s (Inheritance): to the male, a portion equal to that of two females…" {An-Nisaa`, 11} in addition to the following verse, whereas it reads (What means): " if there are brothers and sisters, (they share), the male having twice the share of the female." {An-Nisa`, 176}.
Both verses indicate that the son and the brother of the deceased inherit by virtue of being Asabah and this goes for the daughter. However, this is supposing that the husband and the wife who died in the accident have neither a living father nor a living mother because this isn`t what is mentioned in the above question. If any had a living mother or father, then he/she is entitled to a sixth from the estate of the deceased and the rest is to be divided as mentioned above (The male gets twice the share of the female).
As for the Diyah and the Kaffarah, we have to know the circumstances of the accident in detail in order to figure out whether the husband caused it because he was speeding or it wasn`t his fault and the truck just flipped on them on its own. When these things are made clear, we can rule whether the deceased husband is liable for the Diya and Kaffara or not.
However, if it was found out that the husband has caused the accident, then he is liable for paying the Diya of all those who died with him. This Diya is to be paid from his estate to the living heirs of those who died in the accident. He is also liable for kaffara, which is feeding sixty poor persons, which must be covered from his estate before its division amongst the heirs. And Allah The Almighty Knows Best.