It's in the past 6-12 months that her mental illness has really plunged
Can you be sure if it is indeed the mental illness ? The drugs will exacerbate the illness, I have a son who has a diagnosis of schizoaffective and they can do nothing for his mental illness until he is off drugs, it is possible that a lot of what you are seeing is a combo of the two intertwined.
Sadly, there is nothing you can do to help her my guess having a diagnosis of bipolar myself is she is cycling much faster but I am not a doctor that could be why you have seen such changes recently. No matter the reason only YOUR sister can do anything about it.
Have you tried any meetings for yourself?