You don't have to be a "member" to show up at a closed meeting, but you DO have to be there because you are concerned about your own drinking, not because you want to support someone else. If your sister has indicated any interest in quitting drinking (i.e., she is thinking maybe she needs to do what you are doing), then she would be welcome at a closed meeting. Otherwise she is more than welcome to go with you to an open meeting.
Closed meetings allow for sharing that you might not want to discuss at a meeting where just anyone might be there. You are among fellow alcoholics.