Its hard to diagnose your sister but she very well could be a addict. Hard to tell what she may or maynot be doing. Since we tend to lie so well. Theres definetly some indicators like the picking at the face I've read that prolong usage of drugs like meth and LSD can cause schizophrenia in the brain.
that being said Addiction itself is a mental disease
I personally feel its gonna be hard to help her get better unless some part of her wants to quit.