From what you're telling us, she doesn't seem to be someone who can be reasoned with and you've tried all sorts of approaches. Something serious is going on with her--emotionally. I don't know if it's NPD. We can't diagnose people here, anyway--even if we were professionals. But she is not psychologically healthy, in my opinion.

To my way of thinking, you need to get on with your own life. See about finding a therapist with a sliding scale, who will take whatever a client can afford. Like at a clinic or such. And get some counseling. Dealing with your sis is very stressful and you need to learn ways to cope.

Ideally you and your sis could still be friends, with your own separate identities apart from being "the twin." Make other friends as well. It will be tough, but with a counselor to help support and guide you, it can be done.

I'd like to keep hearing from you, as a fellow twin, too. Okay?