Oh and the forgiveness mantra chanters can never agree on one reasonable definition of forgiveness that doesn't actually just mean ditching those who abused you, and moving on never to bother your arse with them again.
Nobody normal, sane or not insanely cruel would expect you to ever engage with the woman who used to be your sister again if you chose not to - so what would "forgiveness" even actually mean?
If all forgiveness means is to discard them and move on with your life without worrying about them, or letting them cause you any more harm, then great. Do that. No need to wrap it up in a pretty bow and talk a lot of crap about poison and burning to try to sound terribly wise.
Obviously, you don't let rabid dogs back in the house, that's absolutely insane. All those years ago she showed you exactly who she was. The question is, why didn't you believe her?
People who try to claim that not forgiving means obsessing about past cruelties or even bothering about them are the sort of person who lets others live rent free in their heads.
What you should have done was walk away, completely, never spoken another word to the dangerous, harmful woman who used to be your sister again, and just got on with your life. I use visualisations if someone has been an arsehole to me, like pop a little image of them into a box, then stick the box into a freezer (in my mind) or lock it up in a shed. Then you just get on with your life. No need to focus on her at all, she no longer exists to you. It takes a little practice, but it is certainly doable to completely and totally discard dangerous, abusive people from your life, your psyche, your thoughts. I've had to do it three times in my 50 something years, and those three people are like little fuzzy memories of a copy of a copy to me now.
They are gone, and they no longer have any power over me, or interest to me, you could do the same thing.
You can discard treacherous, damaging, dangerous arseholes from your life, with zero forgiveness - whatever definition we're using of forgiveness today - and still not bother your arse with them ever again.
You'll find the ones who wank on about forgiveness are low empathy and low compassion. The sort of people who tell grief stricken widows it's time for them to pull themselves together, you can't mourn forever, or tell women who've just had a miscarriage that God works in mysterious ways.
Dumping the treacherous person's sorry arse forever, not another word to them, not engaging in any attempts to draw you in or gossip about them by anyone else, and any time you happen to think of the harmful person doing your visualisations, diaphragm breathing, meditation etc and letting them blow away like a poisonous cloud in the wind is all the therapy most normal people could ever need.
Unfortunately because you allowed your parents to force this woman into your life, you were never able to exorcise her from your psyche. Time to do just that. She no longer exists for you, as of today. Just discard her, she behaved like trash, and you should have thrown her out with the trash long, long ago.
Exorcise her, give her no more power by engaging even slightly with her, and move on completely. Ah, the air smells fresher already.
No "forgiveness" of any variety actually required.