I have been in a very similar situation with my sister.
She seemed to always go behind my back and tell everyone things that I begged
her to stay between the two of us. This has gone on all our lives.
I finally cut her out of my life for good about two years ago.
It was not an easy decision to make and it broke my heart because I have very little
family left anymore. It did turn out to be the right decision, it cut a very toxic and
triggering person out of my life and I learned that the positives out weighed the negatives.
That is my experience and I hope my story helps you in some way.