"My mother is the kind of person who gets angry at anything and tends to blame others for her unhappiness. My 20 year old sister steals any alcohol she can find in the house and had been known to go through our things to find some."
Are these things linked?
"I cannot help someone who doesn't truly want it."
That's true. Maybe if you didn't try and help her so much, she would help herself. Another thing is, you can lead a horse to water but you can't make it drink. Perhaps the best way to help her is to help yourself. That's when I've been helped the most in life - when other people get their shit together, and either it inspires me or directly helps me. It sounds like she could do with a good example of how to behave. Somebody who gets angry at anything and tends to blame others for their unhappiness is not centered or acting like a star or being responsible for themselves.
Oh yeah. Bully, too.
"Be patient and tough; someday this pain will be useful to you." (Ovid)
Throwing ideas out to you here. Have you seen this?