HappyAdult-
This may not help you, help your nephew directly but I do want to put a little perspective on a piece.
I have been living with an eating disorder most of my life. I have been in recovery just over 13 years (we match for time spent with a therapist).
Though the addiction is different, my inability to "be" present with my own feelings and to deal in reality was very similar to what I see on these boards of people with alcohol/drug addiction (and what I experienced married to on). I used food to numb out instead. I don't write that to say one is better or worse, but to say that your sister might not be in a place that she can actually hear what you are saying....just like anyone in the midst of their alcohol/drug addiction.
Have you consider support in additon to therapy for friends and family (I mean besides here). I found Al-Anon to be helpful for me in many ways to learn people skills in a way I can't get in therapy....it has also been a great help to normalize that other "regular" people have loved ones that struggle with addiction to. It also might be a place to get ideas for your situation.