Hi Peace, Sorry you are having a tough time with sis.
She is very young and has a ways to go probably. You may be absolutely correct about her meds etc, but it doesn't matter a whit!
You might benefit from AlAnon/NarAnon. It would be F to F support for you.
Tell her, you love her, she is fabulous and that you support her.
Now, take a step back, stop analyzing her and mind yourself.
I am assuming that you are also young. Try thinking about how much time you spend thinking/talking about your sister. You might be caught up in the drama (pardon the pun) a bit. She might spend her whole long life being like this. Are you going to spend your life playing backup?
By the way, people here might be tough on you. It is the written equivalent of shaking you and saying, "leave it go! You cannot control it, didn't cause it, and cannot cure her!"
Off topic, just wondering. What are you studying for? Are you going to be an actor or are you interested in some other area?