“You can lead a horse to water, but you can’t MAKE him drink.”
At this point your sister has chosen her path, maybe there’s something her spirit wants her to learn from this, maybe it’s just emotions causing mistakes, but it’s her life and she has to live it as she wishes, you’ve tried and done all you can, anything else magickal is a waste of your energy.
If you want to keep going then I would not use entities, I’d do a poppet binding spell on him to keep him away from her and stop him working against her. A return to sender on his curses done from your own power would not be denied either.
This is my version, but any binding spell you preferred would do this.