You seem to accept that alcoholism is a disease, but you seem to not accept that codependency is as well. Codependents are just as sick. You seem to direct all of your anger to the spouse of the alcoholic. Why aren't you just as angry with the alcoholic? How did your father's drinking affect you? Why is your anger at your mother and not your father? It seems like you're projecting your experience onto this kid, and you think the sober parent has all the power right now, and that may not be true. I'm not sure what the answer is, but I think if you can work on your own emotional health, that can only be good for your niece/nephew. All you can really do is detach from the situation, call the police/CPS as needed, and try to take care of yourself.
I do think this thread is a good thing for us codependents to read. Our kids aren't going to thank us for simply not being alcoholics. We can be as clean as can be and still not protect them if our codependency takes over our lives.