I don't know I have suggestions on disclosure with your sister. I guess I glommed on that because I just disclosed to my step-sister, a survivor of spousal abuse. Because of her deceased father and my abusive mother, we hadn't communicated in 25 years, but at the time she had been my advocate in dealing with their dysfunctional sh*t.
As she deals with separation from an alcoholic second husband, I feel very protective of her. Seems to me you might feel similarly protective. imo, that's good family stuff and some good, positive feelings. (Though my nephew is grown now, I also found myself excited about his accomplishments...more positive family stuff).
Dunno if any of this helps. I do think, especially with the assistance of your T, you'll work it out well.