I buried a two year old cousin who was murdered. Ten years later, the pain is still fresh when I hear stories like yours.
What I have to say is hard but I pray you will listen. You have to forgive, right away. Don’t listen to people who tell you to give yourself time to heal. The murderer’s sin is a millstone around your neck ready to drown you if you don’t let go. You may even find yourself in the ludacris position where the murderer finds Jesus and is saved but you drown because you are still attached to that millstone.
You are not forgetting what happened, you are not condoning what the murderer did, you are not desecrating your nieces’ memory. You are forgiving any debt this murderer owes you for what was taken so unjustly from you, a debt the murderer could not repay.
Free yourself as quickly as possible. Prayers for you and your family.