Mika:
If your sister is a true narcissist, then sadly, I can't think of a successful way to tell her to back off for good. N's just don't hear that.
Again, if she is a true narcissist (and this is very helpful to know) you will have to decide whether you are emotionally able to put up with things not changing (because they won't) or whether it is interfering with your emotional state and with school too much, and you will need to remove yourself from the environment.
If you need to stay at home because you absolutely do not have the finances to do otherwise, or you have other reasons why you don't want to leave soon, no matter how bad it is, then I'd take Bunny's advice.
If things were very, very bad, and leaving was a viable option, if it were me I would.
BT