What is the ruling on paying money to alleviate the burden on a young man who killed his pregnant sister and her fetus in the name of honor?

The killer has committed a grave crime by taking a life that Allah has forbidden, and he must repent to Allah. As for assisting him to reduce his punishment, this depends on the strong presumption that it will lead to his reform. If this is achieved, then assistance is permissible and even encouraged, for Allah Almighty says: "So whoever pardons and makes reconciliation - his reward is due from Allah."