I have a friend from highschool that I keep in touch with. She is an active highly functioning alcoholic, and well, we are just two totally different people at this point in our lives. At one point, she wanted to come live with me and I had to solidly tell her no.
I think as long as you set clear boundaries for yourself, then talking with her sister (who seems desperate for help) might be okay. But lay down exactly what you are NOT willing to do. For instance, if you get invited to an intervention and you are not comfortable with that, simply say no. But if you are comfortable sharing information with her sister, then by all means, share.