Hi, I'd like to help you with your question.
It sounds as if you really have good insight into the situation with your family. I understand it is difficult to stand by and to not only see the situation clearly, but not be able to have any control over the situation.
Although your mother may not yet understand the damage she is doing by enabling your sister, you can start the process of healing by helping yourself. Once she sees what you are doing, she may follow suit. Also, by helping yourself gain a better understanding of the family dynamics and effects of drug abuse, you can be better equipped to deal with your mother and your sister.
If you have not tried this already, I highly recommend Al Non. You can find them on the web at http://www.al-anon.alateen.org/. They can provide support for you in talking with your mother and give you ideas on what might work.
Also, these books may be helpful. One is called The Enabler by Angelyn Miller. Another is Facing Codependence: What It Is, Where It Comes from, How It Sabotages Our Lives by Pia Mellody, Andrea Wells Miller, and J. ***** *****. These are available on Amazon.com or your local library may have them.
I hope this helped you,