AITA for not doing more?

Sorry if formatting is bad, I'm kinda shook rn and on mobile

So my older sister (S) and her boyfriend (SBF) have been on and off for 5 years or so. Out of all the bfs I've met of S, SBF was one of the chillest. We talked about Bitcoin, investing, he was in sales and I'm in tech so we got along so really well and helped each other a lot.

Problem is, when he gets to drinking he can be a bit much. Dramatic, angry, mean to my sister etc. I live 4 hours from them so I just recently found out they finally got back together (which is awesome, he's been really depressed and calling me since they broke up).

Queue up last night, me and my friend are in town, I came to visit family and he wanted to hit up our local events so we got a hotel together to save money. S invites me and my friend to her best friends going away party, so we oblige because something to do and I wanted to see S. well, as expected, SBF is there and gets HAMMERED on shots. I ask if maybe he should slow down and he laughed and said "c'mon you know me". A while later he says he has to go-to the bathroom, S looks at me and says "I have a bad feeling, will you follow him to make sure he's ok?". As a good brother I say sure. Follow him discretely, he walks through the restaurant (we were sitting outside), and goes out the front door. Mind you he wanted to go home, and my friend and I both told him we're getting ready to leave and we're gonna Uber, well just drop you at your house because it's on the way. He wasn't having that obviously.

I chased him down as he walked out to the main road and said hey man just take an Uber with us don't walk!!! I'm not sure if he heard me but he kept walking. I assumed it was just another drunk SBF night, so I turned around and went to go be with S.

S calls me this morning hysterical, SBF got hit by a car last night and was killed.

Im in the middle of a month long vacation right now and I feel AWFUL. my friend is out enjoying the local events and I'm in our hotel drinking beer sobbing. Why didn't I do more? WHY did I just let him walk. I mean, we searched once we uberd from there so we could pick him up and didn't see him, and from the news report of where he got hit it was wayyyyy away from where we drove.

AITA for not doing more? I feel like I just gave up too easy and that my actions of giving up resulted in his death.....the worse part? I leave in 2 days for the west coast, it's a $2,000 trip I've been looking forward to for a year....should I cancel it to be with my sister? She's already a wreck as it is because we just found out my grandpa has cancer. Im just so lost and feel so guilty and I really feel like an asshole......