A little background: I grew up in a tough neighborhood and was physically abused throughout my childhood, with my mother often turning a blind eye. (Because of this, I’m probably overly sensitive to abuse or neglect.) Eventually, I moved away and built a good life for myself.
Recently, I reconnected with an old childhood friend after seeing he had gotten sober and built a family. Through him, I became close friends with his sister. We have been close since December, but I have always lived alone, no kids of my own, and maintained clear boundaries this has never been a romantic or domestic relationship. Though I will say, I have built a relationship, the same way I have with my nieces and nephew with her kids in a short time.

The Saturday before Easter, I went to her 14-year-old son's birthday party. I brought over a water slide rental, a cake, and made breakfast for him. His mother told me birthdays were especially hard for him this year, because he had recently lost his father to alcoholism (literally drank himself to death). I wanted to help make it a good day.

Some family and neighborhood friends attended, including my old best friend (her brother). I brought some alcohol (frozen margaritas) for the adults, intended for light, casual drinking. However, my friend brought his own alcohol and started drinking heavily.

At some point, he began roughhousing with the kids on the water slide. He grabbed the birthday boy and went down the slide with him, breaking his glasses and hurting him. He also threw younger kids down the slide, resulting in injuries one child had clear handprint bruises on his back, and another had welts.
The kids were visibly upset and complained about being hurt.

We tried to get the drunk uncle into an Uber, but he became aggressive, yelling at guests and even the rental company workers who came to pick up the slide. I advised the kids’ mother that he needed to leave, either via Uber or the police, because he was endangering the children.

While this was unfolding, I spent about two hours keeping roughly 12 kids distracted and away from him to keep them safe. I wasn’t involved in the adults' discussions about what to do I was focused on the children.

At one point, I noticed the kids' mother and her friend preparing to leave with the drunk uncle to drive him and his car about 40 minutes away. I only realized what was happening when I saw them heading toward the car.
Confused and concerned, I immediately confronted her (yelling in anger) about leaving me someone who had never agreed to supervise her kids completely alone with 12 children without asking or discussing it with me.

Ultimately, the drunk uncle passed out in his car in front of the house, so they didn’t leave.
Later, after the immediate crisis was over, I had a calmer conversation with her. I explained my concerns more fully specifically about prioritizing her brother’s comfort over her children's immediate safety.

She apologized that "I was upset" which made me feel worse and said she "had to protect her niece" (her brother’s daughter), implying that involving police would hurt their standing with DCF.
She also admitted she "didn't think about me or the kids" when deciding to leave.
I replied, "Well, at least you know the problem."
She responded sarcastically, "Well, I'm just a shitty mother then."
I said, "If you don't put your kids first, yes!"

I left after that conversation, feeling guilty about how upset and the way I acted, and wondering if I overreacted.

So Reddit AITA for being upset after being left alone with the kids during this situation?
Am I the only adult who was rational, or was I irrational considering three other adults handled it differently? Was I the only real adult, or is this a family affair and I should just stay out of it?