AITA if I tell the police/my parents that my sister has been stalking someone for years and might kill her?

AITA? I genuinely feel stuck. The other AITA doesn't allow violence which is how I found this subreddit. So I (23F) have a sister (25F) and I'm really worried about her. Her ex cheated on her five years ago with this woman and ever since she's been obsessed and stalking her. At first she said it was to see if she got karma for what she did, but now it's turned actually psychotic and maybe violent and I don't know what to do or if I should tell my parents.

My sister started dating this guy when she was in high school. He came to our house a million times and we all saw him as a part of our family especially when they stayed together after graduation. Five years ago he cheated on my sister she found out a few days after their fifth year anniversary when they were like 20. The girl he cheated with knew he had a girlfriend but that didn't stop her from hooking up with him. My sister was of course devastated but she gave him another chance and a few weeks later he ended up breaking up with her and she found out that he never stopped talking to the other girl even though she forgave him. When she was exposing the side chick in the beginning on her snap stories I of course understood and supported her as her sister and found the girl disgusting.

I thought my sister would get over it with time and was there for her for the beginning. After two years I was shocked she hadn't moved on. She forgave her ex and even slept with him after the break up but she always hated the girl. The girl blocked her everywhere but my sister still watched all her pages and knew everything she did. If the girl posted herself at a restaurant on her story, my sister would drive to it and eat there and purposely walk by the girls table to make her ashamed of what she did. She also knew where she lived and her work schedule and followed her around all the time and would constantly tell me how much she hates her and how she's never wanted someone to be dead so bad. I found it very concerning and when I told my sister she needs therapy and that our parents should know how badly and long this has been effecting her she shut down completely and told me she was just joking and hasn't told me anything since. That was THREE years ago.

My sister has dated casually briefly since so I assumed she moved on, it has been three years since she told me anything about her stalking and hating the girl and I had completely forgotten about the situation until this past weekend. My sister had a work trip last week and asked if I could feed, walk, and basically babysit her new puppy who needs a lot of attention. I said sure since Im doing all my classes online this semester and stayed the week at her apartment. It's a few cities away so I've only gone to see her a few times since she's moved here. And duh as a sister I made myself welcome in her apartment and slept on her bed. I always slept on the left side but on Saturday I was cold and wanted to sleep on the right side which is closer to the heater and when I laid my head down I felt something uncomfortable and realized there was something inside of of her many pillows. Inside of it was a composite notebook labeled "Anna" (not her actual name, but the girl her ex cheated on her with). When I saw that I was shocked. I hadn't thought of the name in years.

The journal was so scary. Basically she never got over the situation and has been waiting on karma to get the girl back for being the other woman but it hasn't. I didn't keep up with the girls life but she moved away years ago, got married, and makes lifestyle content for what looks like fun since she doesn't have many followers on any of her pages. Her life seems perfect. My sister goes on and on about how much she hates this woman but what scared me is that my sister FLEW OUT to this girls city to stalk her in person and figure out what house she lives in... she tried to figure it out from what she posts of her house and area, during a time she told us she was visiting her friend who was sick. I was so shocked she would go that far. And it wasn't just one time, she's done it twice from what I read. Also the notebook I found is dated and only goes back to May of 2024 so I assume she has multiple other ones none I could find. She's stated in it the only way she will get over it is if the other woman suffers and since karma hasn't gotten her back my sister will. There was also a lot of other weird threatening things in there like how she describes wanting to watch her pass away...

I was so weirded out and when my sister got back Monday I tried to act normal. I know I'm crazy but a part of me thought she left the state to k*** the girl. I went to view the girls Insta page on a burner I made and she's posted since so she's alive. That's when I realized that fact that I could think my sister would seriously do that is insane. And that if five years later she still can't get over the situation she needs a lot of help. I'm worried for her and also the other girl. I never thought I could feel bad for her but if only I could describe better what my sister wrote, I truly feel like she may be in danger. The issue is I'm scared if I tell my parents who don't know anything and that my sister got over it way before I did, there's not much they can do just based on my word alone. My sister has a job, apartment, her own life. I didn't take the notebook I obviously don't want her to know I know so I have no proof. I wish I took screenshots but I was so in shock all I could do was read and when I finished I put it back and felt dirty. I don't know what to do. WIBTA if I DONT speak up? Should I try to go back to get more proof and then tell my parents so they can maybe stage an intervention? Im so stressed.