Is the best course of action revenge or cutting someone out of your life?

So, this person isn't exactly related to me but rather my significant other's siblings. So, my step sister. For a very long time, and I mean years, she's treated me like shit and there has been countless attempts to try to fix things but it started to become draining and it felt like I was in this vicious cycle. Whatever problems she has, certainly didn't derive from me, but I don't understand why she has to also drag me into it. Long story short, we got into a vicious argument while I was home alone and she till this day, denies the argument. I left a week after the fight but I still had my belongings staying at the place of residence because I have to return a year later for school.

I left about 10 months ago, so it was pretty odd for me to find her posting a picture of herself with her face cropped out and wearing one of my dresses. And it actually turned out that more and more of my clothes where discovered in her room...10 months after I already left the country? I do my own laundry and I organized my closet before I left and now it's a wreck. She denies it all and only tells me she found it in the laundry basket... but I left 10 months ago. She refuses to tell the truth and is also giving me sass with responses such as "I posted the picture because I liked it and when you like something, you post it". But, that's the only photo she ever posted.
Anyways, she left my messages on read and is completely ignoring me and out of anger, I blocked her. I will gather my belongings in a few weeks but I'll just donate them as I don't feel comfortable to wear them anymore. I was also considering donating her stuff too for her general behavior.

Talking to my significant other and their parent is ruled out because for years, we've tried every approach with her. But there's never been any consequences or acknowledgement of wrong doings. Nor mutual respect. They're so used to getting an excuse or lie from her and moving on. So what can you suggest that I do differently?
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Oh shoot, excuse my English. I meant she is sister in law
Is the best course of action revenge or cutting someone out of your life?
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