AITA for abandoning my sister

Hello, first time poster so I’m a little nervous.I’m 21F and my sister is 17F. We have always been incredibly close, as we grew up in an abusive household half the time (our parents had joint custody and our dad is an alcoholic).Essentially, I have always been there for my sister. I tried to take the brut of the abuse when we were little and was always the one cleaning, cooking dinner, and dealing with dad until I was 16, when our mom finally got full custody. I think because of this, my sister sees me as a third parent. She was my reason keep on pushing, even when I was at my lowest. She was the most important thing in the world to me - her safety and her security were things I would do anything for.I recently moved about an hour away from my moms house to go to college, but my sister and I would talk all the time. We both struggle with mental health issues, but we’re both working on it. We both go to therapy and see a psychiatrist (separately of course).The problems came about when I noticed she was answering my calls less and less. At first I was totally fine with that, I can’t expect anyone to drop everything for me 100% of the time. However, her reasons for ignoring me started to feel like she was blowing me off. One day I called her when I was in a really bad place (after trying to get in with my therapist for an emergency meeting) and she basically told me to go bother someone else. That she was about to leave to go see her friends and that she didn’t feel like dealing with me. After I calmed down I texted her and said what I’ve said here, that I felt like an inconvenience to her. She said “lmao. You don’t want to start with me.”We argued for a bit but she was being so hurtful that I told her I needed her not to contact me for a while. A few months go by, and she FaceTimes me, demanding me to apologize saying I ruined our relationship. I laughed (nervously, but I realize now that it totally didn’t come off that way) and asked her how. She starts screaming at me, saying I’m just like our father, if she ever sees me again she’ll kill me, she’ll beat the sh*t out of me, and then she picks up a razor and says “you caused me this much pain, now you get to see the results.” I won’t put exactly what happened but I’m sure you can guess.I immediately hung up and called our step dad who happened to be home, begged him to take her to a psychiatric hospital, but he told me it was not my place to make that call and that I should just leave it alone. I blocked my sister on everything because she traumatized me and I’ve been feeling insanely guilty ever since. My mom mentions to me every time we talk that I need to grow up and stop being so petty. My therapist has told me this is an example of a healthy boundary, people can’t treat me like this, and I don’t deserve this, but I feel like I’m abandoning my unstable baby sister and my moms comments don’t help.AITA? I feel awful and I miss her but for the first time in my life I want to put myself first. via /r/AmItheAsshole https://ift.tt/30NEoNu