Is it permissible for one of the killer's brothers who is able to do so to pay the blood money to the victims' guardians to spare him from execution, without incurring sin?

The ruling for intentional, aggressive killing is qisas (retaliation in kind), unless the guardians of the victim's blood pardon it, either for compensation (blood money) or freely. This is based on the Almighty's saying: ﴿Prescribed for you is legal retribution for those murdered﴾ [Al-Baqarah: 178], and the Prophet's (peace be upon him) saying: "Whoever has a person killed belonging to him has the choice of the two best options: either he is paid blood money or he is given retaliation."

If the guardians of the victim's blood pardon the qisas, there is no harm in assisting the killer to pay the blood money if he has repented to Allah and if his assistance does not encourage aggression. It is permissible to assist him from zakat funds if he is in hardship and among the gharimin (those in debt), provided that he is not known for corruption and evil, that signs of repentance and remorse are apparent on him, that intentional killing has not been repeated by him, that he does not possess wealth sufficient to cover the blood money, and that the reconciliation is not exaggerated.