3 years ago, I cut my brother off. It was horribly difficult for me. My brother has been physically and verbally abusive to me since childhood. I tried to mend fences with him, but I had to give a mile, and he wouldn't even give an inch so it failed. I mourn the loss of the brother I wish I had, but do not miss the actual treatment I received from him.

Ending a relationship with a family member is a huge deal. Most therapists discourage it if at all possible. If your therapist is encouraging this, there is good reason.

Regarding your sisters issues (and it sounds like there are many) she might and probably has something diagnosable. That doesn't excuse her behavior. I would be direct with her on why you are ending contact. Maybe it will make her think about her behavior. It might take years, but your honesty with her could provide her with insight to become a better person in the future.

If your life would be more stable without her in it, you have all the reason in the world to cut contact. I would do all you can to preserve the relationship if its salvageable, but if she brings you misery, you are better off freeing yourself from those dynamics.

Best of luck and hugs!