I don’t have enough information to know if your sister is sociopathic. I do know that regardless of whatever label you put on it, you have good reason to be afraid of her. Her behavior toward you is more serious than the usual sibling rivalry. People who are as temperamental as you describe her are often using their changing moods to control those around them. Ironically, it is often because they are insecure.
It probably isn’t all that helpful to try to define a diagnosis. If you want a relationship with her, you are old enough now to attempt to have an honest talk with her about the effect of her past behaviors and how to move forward. You will be more successful in such an attempt if you resist the temptation to blame and shame and instead focus on what you want from her now.
If she continues to be volatile and isn’t interested in talking, there isn’t much you can do except to stay away from her as best you can. There is no need for you to put up with abusive behaviors just because she is your sister.
I wish you well.