From the tagged image in this post... It looks like the autopsy found reason for homicide due to multiple stab wounds leading to her death. I would reach out to their office-- call or visit in-person. I doubt the cornerer still is working there but the worst they could do is not tell you anything which leaves you back to square one but I think worth a mention or effort. See if there is a way you can look at her autopsy report or any death certificates.
I know this was a long long time ago so if you don't recall that is totally normal. But do you remember your cousin being there or present at all that day? Have you ever spoken to him or know any information within his dynamic?
You also mentioned being present during the time of the attack, I am so sorry you had to bear witness to this heinous crime. I know the mental & physical tolls that can come from various traumatic incidents. Could you expand more on what you remember happening that day? I know it seems like none of it will be relevant but you could be very surprised by little elements creating a timeline or mental picture.
Do you remember an argument breaking out? or maybe being sent outside or to your room for a parent/adult discussion? Did the attacker break inside the home or did he have a way onto the property with little to no questions? I only ask because you mention a dog, Sadie. I have grown up with a multitude of different dogs and have worked in behavioral training. From your initial post you say you and the dog were said to be taken outside due to the nature of the crime and your age. I wonder if the dog did any barking or growling/howling at the attacker? It doesn't seem like it if you and the dog just went out the back door (seems like the dog is comfortable with the attacker, meaning dog wouldn't give barking signals to a person they know/comfortable with). Again, I am so sorry for your loss and we are here to help you find the answers.
Paulding County Medical Examiner & Coroner Contact Information
Name: Paulding County Medical Examiner & Coroner
Address: 160 Confederate Avenue, Dallas, Georgia, 30132
Phone: 770-372-8596