See, I have a different perspective.
I’ve been estranged from my little sister for 7 years. Yes, there was an ‘event’ where she did and said some truly unforgivable things, knowing I was in the most vulnerable position of my life. But that isn’t why we’re estranged.
We’re estranged because she’s a person who is currently incapable of considering other people, thinks nothing of trashing relationships, prioritises her merest whim over anyone else’s dire need. She’ll never be there in a crisis, she’ll disappear when she’s having fun and cling fervently to the denial and misplaced anger inside herself rather than getting help or trying even the smallest exercise in self-awareness or personal growth. Like your sister, OP, she has form for trying to reconnect by pretending (and insisting other people pretend) that nothing ever happened.
I came to the conclusion that life is hard enough without people like her in it, or in my kid’s life. She can peddle that drama and immaturity elsewhere.
Technically, I forgave her that terrible night. I just choose not to associate with her. In my mind, I leave a tiny door open in case she makes inroads to change her life through therapy. But I’m certainly not holding my breath. I hope it for her sake, but it’s increasingly less relevant to me. 7 years on, I’m the better for my decision. Her ability to hurt and neglect people is still alive and well, but I’ve left that battlefield far behind.
I have a very strong sense of family, and family responsibility. I don’t expect much of anything back, but even I had my limits. I see it as a big self-care move on my part, a decision that I am worthy of love and care (not just someone who dishes it out).
I didn’t write a letter or have it out with her. I told other family, though, so I suppose it got back to her. I just calmly cut contact. Not much drama ensued, after all I’d just followed her lead of generalised neglect. And I’m all the better for it.
Sorry for your losses, may new brighter days be ahead.