You're right that nothing will ever be the same. The only advice I can give is to be good to yourself and your loved ones. Take extra time to take care of yourself, pay extra attention to your needs. Particularly when a loss is sudden and violent, its easy to focus on all the questions you must have. Focus on your cousin. Her life, your memories. When my friend was murdered I found writing to be really helpful and cathartic. It allowed me to think and talk about him in a safe space, and left me with a beautiful record of my memories of him.