(Don't put any over complicated potions.)
Love this addendum.
Is your sister seeing a therapist? Regularly? Is she on medication that could be altered to fit her current emotional needs?
First things first, bipolar disorder and PTSD are medical challenges and need to first be addressed medically by a professional. No exceptions.
Beyond that, general banishings of negativity and drawing in of positivity (as stated earlier) are what come to mind.
On a mundane level, just keep engaging her socially. Keep her in the loop, let her know she's still noticed, loved, appreciated, and connected with you. PTSD is a b**** and coupled with depressive episodes can make it downright horrible. Sometimes there isn't much you can do besides just be "there" for them, be present and understand that it's not the person's identity that's behind all of the symptoms. That they're still there somewhere, feeling lost and overwhelmed and really just needing a landline.
But there isn't much you can do to ease those symptoms without professional interference from the beginning. Look into group therapy if you haven't already. It tends to help a lot with PTSD patients because one of the major symptoms is that feeling of overwhelming isolation once you become lost in your memories and instincts. You need people you can connect with who really understand where you're at and you can only really understand it if you've gone through it yourself.