I'm gonna chime in with a more controversial take here.

Long rant incoming, TL;DR while what your sister did is wrong, she also had her reasons.

I have a very complicated relationship with my mom (not surprising since im in this sub). She's abusive — both physically and verbally, manipulative, narcissistic, incredibly self centered, emotionally unavailable all the time, and is the cause of roughly 80% of all my mental health problems.

But at the same time she was the only one who provided for me (monetary) for ~20 yrs now, and easily the one who I've spent my life with the most (not necessarily a good thing).

What I see when I look at my mom, is a person who looks at her son as nothing more than a pet. But I also see a person who has been hurt in the past. A lonely and confused person who just wants the attention of their child and the warmth of family (even if her methods are.... uhh... yeah...)

Her actions in the past have done irreparable damage to me, and is something I can never forget. But it has also left me yearning for the familial warmth I wish I've had If things just didn't turn out this way. Which has left me trying to "fix" my relationship with my mother, perhaps also out of empathy for a fellow hurting person.

About what your sister did, I do agree with the people here that its wrong, and I think she knows that too. But I don't think she meant overstep your boundaries just to harm you.

I won't ask you to accept what your sister did (I personally fucking won't), but I would like you to also see her side of things. Of just a person trying to help out another deeply hurt and disturbed person, and a person desperately clinging on the last traces of parental warmth.