British Columbia, Canada
I am glad you posted here as you do need some support in what you and your siblings are going through. We can't begin to offer you the kind of help you are needing but can sure steer you on the right path to find it.
First off, your Mother didn't intend for you to become the parent to your brother and sister, but that is what alcohol does sometimes. I am sure she loves you all, but needs help beyond what you are able to give I am afraid. Before this situation robs you all of your youth, I too suggest you speak to the guidance councillor at your school, or go to the local church for guidance. They are there to help you, and will.
I realize you love your mother and feel telling someone might hurt her, but by sharing your life's story with the right people you only stand to improve the quality of life for all of you, including your Mom. It is never easy to deal with someone with an alcohol problem, and certainly not expected of a teenager to try. My thoughts are with you in your struggle, and let us know if you get help for you, your sister and brother won't you? Keep posting if it helps you keep your head clear and we will be here to support in whatever way we can.