Am I The Stupid One In This Scenario?

I had a question for the internet because, even though i might already know the answer, I'd still like advice from strangers.. It's the most accurate that I could get it, at least from my POV. It is very complex and I digress.. but ok. So, my younger sister/ best friend (27) had gone last week to a dinner with her ex-coworkers. One of her former co-workers had gotten a bit too drunk and needed an uber home, my sister (out of a job) called me and asked for $50 since she was going to be overdrafted, and would need money to get home, I sent it.. I had texted her around 5 in the morning asking if she was ok? She answered and said that she was sleeping over another ex-coworkers house (who was a good friend to her).. this lady lives no more than 30 block away from where we live *$20 max on a busy hour* and had work the following day. I know because we used to have Thursday dinners, and we'd pick her up from work. Im straying from the story.

So she (my sister) given that she can't really hold down her liquor, I get paranoid that she doesn't know how to compose herself. It could be that I'm not giving her too much credit, however she can get very messy when she can't handle it.. anyways I had asked her if she was OK at 5 in the morning. She says that the uber was going to be pricey. I checked the uber app and see that it is only $11 from this ladies house to the home. I had given her $50 mind you..

Alright to the knitty gritty so She had a one night stand with her higher-up, essentially the manager about 3-4 years ago, supposedly nothing serious.. However, they've had a flirtatious thing going on for about 2 years while she was working there (the 1st time). He (approximately 32-ish idc) had been in a relationship for about 15 years during all this. Apparently toxic, lived together and they fell out of love, yadda yadda.. at some point during their end of the relationship he cheated on his gf with my sister, (no sex) buuuut my sister couldn't work with him anymore because of his constant mood swings and she quit working there, not once, but twice! Like a year apart!! He was pissed off with his life, verbally took it out on her during their shifts at work she didn't like it so she left. BOTH TIMES!! She didn't want to accept that it was him that was the issue. WHATEVER! When They finally saw each other again after like idek a year maybe *outside of her passing by the ol' job* ( it's a liquor store btw) they haven't interacted really. She ended up spending the night at his place the night she went out, and she just came home saying that "things took a turn" and "it escalated way faster than what I thought ".. sooooooo I want to know am I the a-hole here? I don't like the guy, from the stories I've heard from her mouth, and the mouths of some of their coworkers he seems sociopathic, and tbh I kind of feel bad for this guy's ex-girlfriend. Like how does it look to be "I guess, dating" a former co-worker? You know.. It seems highkey suspicious! It seems like they've had a thing the entire time they've worked together, at least that's what I think. I mean if my boyfriend and I broke up and he goes on the be with his coworker... mmmmmmm SUS AF!!

This happened last week and I haven't talked to her since I found out she's now seeing this guy.. she confronts me today and asks what my problem was. I have a really bad habit of not communicating my feelings to people. No matter how close we are.. I end up just telling her that I'm embarrassed for her about this situation and that essentially was the final straw for her and now our relationship is officially ruined. We aren't "friends" anymore. Yes I'm sad, but I guess I have no choice but to feel this way for a little bit... No more hanging out with her or her friends, who have become cool with me. I'm cut off from her life, and I don't really have friends of my own, so I rarely have incentive to leave my house. I'm very introverted, and don't talk much to people outside of my circle. I ruined my relationship with the one person I truly care for besides my parents, over a freaking stupid man, who I know nothing about. But honestly don't care enough to get to know. I'm a stubborn idiot.