An off-the-wall question, jexa: has anyone ever suggested that your sister might need treatment for Borderline?

I've met some wonderful people here at PC who report having a diagnosis of BPD but don't seem to fit any of the Borderline stereotypes. Your sister's volatility, rages and "Jekyll/Hyde" transitions fit a lot better, and your "walking on eggshells" reaction is just about a trademark of BPD. It's even the title of a book: Stop Walking on Eggshells: Taking Your Life Back When Someone You Care About Has Borderline Personality Disorder. I haven't read enough of the book myself to either recommend or discommend* it, but there's lots more about it at Amazon including reviews and sample pages.

Your sister's drug problems might well turn out to be on top of, or the result of, something else -- such as BPD.

Thanks for sharing/asking, and best of luck!

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*Real word!