Your therapist probably mentioned "Cognitive Dissonance" to explain your sister's behavior. In this phenomenon, someone decides how they want to view the world, and become increasingly uncomfortable when evidence arises that disproves their view of the world.
A classic case is a woman who doesn't want to believe, despite all evidence, that her husband is having an affair. That woman will fit new evidence into the puzzle in such a way that she can still tell herself her husband is faithful. The stronger the evidence, the harder she has to work and the angrier she is at the messenger, often cutting them out of her life and calling them a liar.
In your sister's case, she has decided to believe she had a loving daddy and normal childhood. In the face of the clear evidence that you were witness to her father raping her repeatedly, and you remember it, her only recourse is to kick you out of her life so that she can continue to frame her fantasy childhood the way she wants to.
She doesn't blame you for the fact she was abused - she has no choice but to remove your voice from her reality. If you were a less reliable witness - if she could simply dismiss you by saying no you're wrong, you weren't there, you didn't know him, etc. she could continue to have you in her life and continue her construct of her childhood.
But she can't. You were there, and she knows it. With cognitive dissonance, she has no choice but to completely dismiss you because the alternative of living with the knowledge that her father raped her repeatedly is too painful.
Best wishes for you. You have nothing to feel guilty about.
I think in your shoes I would discuss it with a therapist. A professional would be able to answer your questions about whether there is a tendency to blame other family members who were around when bad things happened. You might never regain your closeness with your sister -- ask the therapist about the tendency of some families to gloss over the bad things about their parent once he dies even when he was horrible. (I have some cousins who have glorified their alcoholic dad who made their lives a constant uproar, because he is now safely dead and they want very much to feel important and to feel like their lives weren't a mess when they were kids. It's delusional but it might actually be something they need.) Anyway, try to let go of the worry that your sister blames you. She might blame you and she might not, but unless something changes in her mindset (and you can't control that) there is little enough you can do to change her mind. Talk also to the therapist about that.