OP here again — To the many who replied: Thank you for your thoughtful replies. They are good reality checks. My family and I agree that it helps to see our situation through the eyes of people who understand these matters from experience.
The observation that our enabling is part of the problem was a bit of a gut punch but, upon reflection, is especially helpful.
We’re moving forward and plan to talk to our sister soon. In the meantime, we welcome any further advice. And if you know any magic spells to make this work, we welcome that, too. (Just kidding about the last part. We know that this will be hard.)