He will have an attorney, may be a Public Defender, but he will have an attorney, whether he is tried as a juvenile or an adult.
What happens to him, is out of your hands, and your sister's hands.
You can continue to stress and worry over him and his situation and there will be an outcome. You can turn your focus on your own children and family and guess what? The outcome is going to be the same.
Take some more slow deep breathes. Do this everytime you start to worry about this, it does help, honest.
Love and hugs,