Do yourself a favor and go to Al-Anon and at least get some of their literature to read. I've been enormously helped by Al-Anon. Reading the literature does not make going to meetings unnecessary, but it can be real eye-opening.

Your concern for your niece is commendable. The situation warrants concern. It is very frustrating that you have really no control. And you don't! That is the first thing Al-Anon will try to teach you. Continue to love your sister. Continue to love your niece. At some time in the future, you may become even more important to your niece than you are now.

I highly recommend going to a meeting. Try to identify someone there who is seasoned and mature in "working the program." Ask that person to be your temporary sponsor. Ideally, go to some different meetings and identify several persons who impress you as calm and mature and maintaining some "serenity." Get some phone numbers, and talk over your perception of this problem with some experienced members. Don't let shyness hold you back. I am very shy, socially. Desperation, however, is a great motivator. When you really love and really care, you do become desperate. Let it lead you to improving your "perception," which probably needs to change in ways you can't now understand or anticipate.