What is the ruling when one son kills his father and another son kills his mother?
General Chapter
Al-Mughni
Book of Wounds
Primary text
If the marriage between the parents was valid at the time the first son killed the father, then retribution (Qisas) is only obligatory upon the son who killed the mother, not the son who killed the father. This is because the second victim (the mother) inherited a portion of the first victim's (the father's) blood money. When the mother was killed, the killer of the father inherited that portion, meaning the father-killer inherited a part of his own blood money, causing Qisas against him to lapse. Consequently, Qisas becomes due upon his brother (the mother-killer). If the father-killer then kills his brother, he inherits from him, provided no other heir exists, because he killed rightfully. If the father-killer accepts blood money (Diyya) instead of Qisas from his brother, the Diyya is due, and they settle the difference between them; any surplus belongs to the one with the greater due from the other.
Supporting text
If the marriage between the parents was not valid at the time of the father's killing, then Qisas is obligatory upon each son towards the other. If one acts first and kills his brother, he has satisfied his right and Qisas against him is dropped, as he inherits from his brother since the killing was rightful. This inheritance is not prevented unless the deceased brother has a son or son's son who screens the killer (the paternal uncle) from inheritance; in that case, the son has the right to kill his uncle and inherit from him if he is the sole heir.