Hello and thanks for visiting JA.
I think you know this - but she is probably NOT going to change her ways, and she is not going to be more accommodating to the family as things are.
That leaves you effectively two choices - either you go an as you are, having cut off contact with her, or you bow to your parents wishes. Unfortunately, only you can decide which is the best long term choice for you.
If you go along with what your parents want, what does it mean for you - another forty years of strife?
By cutting her off, you have already stopped enabling her and empowering her, and if I were in your place I would not for a moment consider going back on that unless and until and until she changes.
If there is to be the slightest possible chance of her changing, she needs to be confronted with your feelings about her behavior, and made to understand that, while you care for her, her behaviour is unacceptable and will not be tolerated, at least by you
He also needs to understand that that any continuation of this behavior will have consequences. They need to be spelled out to her very clearly, with clear emphasis on the fact that they will apply immediately.
We humans only indulge in behaviour that brings reward of some kind. Only when that reward (whatever it might be) disappears, or the consequences of our behaviour promise to be unpleasant do we consider changing what we do. She needs to be given reason to change, and if you can get your parents behind this, the better the chances of success.
Ask her too, what he is prepared to do to change her behaviour in future – tell her to research what might help her, what professional help she might get, and even consider a ‘contract’ between you. In other words, involve her in her own change, with a prospect of a small reward for success and dire consequences for failure.
However, don’t get angry, stay cool and in control, matter of fact and stick to the facts. Avoid drama.
If her parents come on board with this approach, there may be some hope.
If it fails, then we are back to the two stark choices.