Hi,
marigold14. Welcome to BPDFamily.

I’m sorry that you had to find us, but I’m glad you have. We’re a peer based community and most of the members here are dealing with something similar to your situation. We get each other.
First and foremost, your sister’s behavior is unacceptable. I would agree that there is something very adverse going on with your Sis. It’s interesting how you were able to provide positive reinforcement and stability for her. Covid-19 is a very good reason for a lot of people to act out, or so it seems. I don’t buy that, but it makes sense. She dumped it on you. She couldn’t hold on to her emotions anymore and the pandemic allowed her to let them loose on you and your husband. The 5 page letter wasn’t fair within the scope of what she’s trying to prove. A calm and collected conversation is fair. She’s most likely not capable of participating in something like that.
So, what do you do? You love her and rightfully so, but to what extent? The love in your heart for her is real, but you have a life of your own now and that should come first. There are tools that will help in managing the abuse that your sister doles out. There are a lot of resources available here for that.
It will always come crashing down with her unless she decides to become self aware and put the work in.
This post is about you. You’re feeling bad about the situation, but, at the end of the day, is it really your responsibility? You have a husband. Do you have kids? Important question when considering the circumstances.