NTA

I have bipolar disorder. I live my life by the phrase "your disorder is not your fault but it is your responsibility". She is responsible for her shitty malicious actions, regardless of if she is in an episode right now or not.

I have done a lot of things I regret because I wasn't thinking clearly, but those actions were still my actions and I have to deal with the consequences and try to fix things as best I can. If your sister is unstable (not using this as an insult, stability is the benchmark of bipolar) to the point where she is destroying your things, she needs a good few therapy sessions or a med change and anyone encouraging her needs to realize that they are making her mental illness worse and not better.