Hi CornishSelkie

welcome!
I feel the pain in your post, and your love for your sister. It is so hard when people we love just cut us out completely. It does make you question everything - I can so relate.
People with BPD tend to see things in black and white, you're either all good or all bad. It's possible that when you defended your mom, your sister's abandonment fears were triggered. She painted you all black as a defense, making you out to be a bad person, abandoning you before you could abandon her? Does that ring true? It doesn't make sense to someone who doesn't have BPD, but their thoughts follow a different pattern.
It doesn't happen overnight, but it's ok to let her go. You can't force her to feel any differently than the way she feels right now. She has the right to make the decision she's making because she's an adult. The best thing you can do for your sister is heal. Take time to recoup, do things you love, and feel whole again. Get healthy and well so that when she's ready to talk, you'll be ready.