Is retribution (Qisas) obligatory if a parent kills their co-parent when they have a child?

Primary text

Retribution is not obligatory if a parent kills their co-parent when they have a child who is an heir. This ruling applies whether the child is male or female, or whether the deceased has other heirs sharing the inheritance. The reason is that if retribution were obligatory, a portion of it would be due to the child. Since retribution cannot be obligatory upon the child for an injury done to their father (the killer), it cannot be obligatory for the injury done to the other parent. Furthermore, if only a portion of the retribution were due, the entirety of it falls away because retribution is indivisible, similar to when some rightful heirs waive their share.