alcoholism is recognized in the dsmv, and by the ama, as a disease.
think: dis - ease.
even diabetes and some cancers can be triggered by our own actions. once you have one of them, it is a disease as surely as one you caught by unprotected sex, smoking too many cigarettes, or a whole host of other "unnatural" forces that bring it on.
your anger is understandable. if this is what helps your mother to accept her affliction, her curse, her bad choice, then so be it. it may be a form of denial on her part, but you all want the end result to be that your sister has a chance to get well. i hope someone can have her committed.