I think it's a bit silly to have her on those 3 types of medications at once and it makes me question what exactly are the medications. Many of the medications have multiple uses, such as a mood-stabilizer also functions as an anti-psychotic, an anti-psychotic also functions as an anti-depressant and so forth. Unfortunately, many of the anti-psychotics do make one feel pretty awful, sluggish and can even have cognitive effects. I only know of ONE anti-psychotic that does not have strong sedation or cognitive effects: sertindole.

Newer anti-psychotic medications strive to reduce side-effects but it's a real challenge. There is a lot of interpersonal variability as you mentioned so some people become a guinea pig until they find the one that "works the best". It's a very tough mental disorder and my sympathies go out to your sister. However, it comes a point where you can over-medicate a person until the symptoms go away and cheer in joy, or there's this, symptoms are gone but she is way over-medicated. I don't know if she's also on very high doses of the medications but I feel the doctor has to re-evaluate the medications because they're not improving her quality of life.

I won't lie to you, there are different types of schizophrenia and some have poor prognoses. The good news is paranoid schizophrenia has the best prognosis. Can the illness get worse? Yes it can. Although I'm not a physician, her treatment is unacceptable. One indicator of whether the illness is getting worse is to do brain-imaging scans, usually MRIs because they give enough needed detail. There are more detailed ones but they become more expensive and less available for public use. She may also have EEG or even MEG tests to evaluate her neural functioning (MEG is a newer and more detailed version of an EEG). Some patients can get worse with time to a point where stronger medications are needed but at the cost of greater side-effects. I'm not saying this will happen to your little sister, it may not.

In the end, since she cannot go by herself to the doctor, you'll have to go with her. If the doctor doesn't want to change her medications, I'd consider going to another one.