It is difficult to watch someone you care about suffer. As you mentioned, you have attempted to make your cousin aware of the consequences of her dangerous behavior yet it has not stopped her. She most likely does not fully realize that her behavior is destructive.
I would encourage you to continue to voice your concerns to your cousin. You may also want to discuss this matter with your parents or hers. Perhaps one of your parents would be willing to call her parents. If you chose to speak with her parents, you do not have to “tell on her.” You could alert her family about your concerns without being specific about her behavior.
Another idea is to speak to a faculty member (guidance counselor, etc.) at the school about your concerns. In most areas, school is not in session at this time but it will be in the very near future.
You may also want to consider approaching her accompanied by other individuals who share your concerns. Approaching with a group of individuals who all share the same opinions might help to further strengthen your case.
The bottom line is that you can’t control someone else’s behavior. You can and should continue to state your concerns but realize that there is a limit as to how much you can affect someone else’s behavior. That is the unfortunate reality. Please take care.