The American Psychiatric Association considers alcohol use to be a problem when it significantly affects occupational, social, and/or recreational activities. An alcoholic is someone who will have a drink, even when they know it will lead to them getting drunk, and they don't really want to, or they will get drunk even when they know bad shit has a good likelihood of happening. OP, does this describe your sister?
If it does, the best thing you can do for her is let her experience the consequences of her actions. This will negate any benefit she gets from compulsively drinking, and she will (hopefully) one day wake herself up to the fact that she needs help. As others have said, you can't do it for her. Also, this keeps you from getting into a pattern of "saving" her. It's a pattern you want to stay away from, because "saving" others (especially women you are close to) may feel good at first (gee, like drinking), but will eventually mess your life up. You're already doing good for yourself by setting yourself a foundation before going out into the world. Keep it up.
Good luck to you, your sis, your mom, and anyone else involved in this.