What is the ruling on Qisas when two sons kill their respective parents, who were married to each other?

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If one of two sons kills one of their parents (who are spouses), and then the other son kills the remaining parent, Qisas is waived against the first killer and becomes obligatory against the second killer. The reason is that when the first son killed his father, his brother and mother inherited his estate and blood money. When the second son killed the mother, the father's killer inherited from her. Consequently, the first killer inherited an eighth of his own blood money, which waives Qisas against him. He has the right to Qisas against the second brother, and he inherits from the second brother according to the apparent position of the Madhhab if Qisas is executed.