Thanks everyone.
The thing that is strange (not so strange, actually) is that my entire family is not unfamiliar with co-dependency issues, addiction and recovery. We've all had it touch our lives in one form or another. And we all know this about one another, so there is no secret. But my sister likes to believe that she's the only who has ever experienced anything so horrible or had to deal with this issue and the effort of detaching.
My feeling, on an intellectual level, is that they've had their heads buried in the sand for so long and so deep that now they are in a panic themselves. They have been attending Al anon constantly, which is great and I'm glad for it - it will benefit everyone. So, I am trying to use my detachment skills in this situation just as I have with my H in his sobriety and with difficult child, even easy child.
I can continue to offer support in loving ways by simply asking my brother in law "How's everyone doing?" and leave it at that. At some point, maybe I will get to visit my nephew during one of the designated visiting times, but I don't think anyone should visit for a while - he needs to settle first. But I would like for him to know that I'm thinking of him and praying for his well being, which I'm sure he does know. I just hope that my sister's twisted views don't get in the way or skew his and my relationship, as it's threatened to do in the past. She won't let anyone have their own relationship with any of her 3 kids - she's threatened by other's knowing her kids more than her and always has been.
Thanks for all the support - despite my own efforts to detach, it still hurts.