I choose Other.
Here's my thoughts.
They are to blame.
They are the ones who choose to leave rather than wait. I can understand that they didn't want to stay out much later, but that doesn't mean they couldn't call a taxi from where they were and have it pick them up. I'd assume that one of them has a cell phone, or that there would be a phone available at the bar. There was no reason to walk. Though, if you were only planning to wait another 15 minutes, they had no reason to insist on leaving. By the time they got a taxi, you would be ready to leave.
You attempted to warn them, and rather than listen, or politely disregard your advice they called you a victim blamer and yelled at you. This may be unpopular, but sometimes the victim has to take responsibility for their actions. Especially when those actions needlessly place them in danger. They are adults. They are responsible for their own actions. Drunk or not, they made the choice. And getting drunk was also their choice.
The attacker(s) are to blame.
They attacked these people. They didn't have to. Without their needless actions, this would have never happened. So obviously they are more to blame than anyone. I don't think I need to explain this position further.
You have no fault.
You did the best you could given the situation. You can't control all three people at the bar. You're not their keeper. And you did warn them beforehand.
Your brother may hold some fault.
I can't say for certain without knowing all the details, but he was the catalyst for it all. He choose to get so drunk that he wasn't "driving safe" (assuming he risked vomiting more), and was most likely the one that delayed your departure, in the first place. He needs to hold some responsibility. And probably needs to, at the bare minimum, slow down on the alcohol. Wouldn't hurt for him to apologize for his behavior...
Keep in mind, I have very little information to go on, but seeing the situation as it was describing in the opening post, I feel the blame largely falls on the attackers, with the girls behavior being needlessly reckless, and the brother's apparent abuse of alcohol being the catalyst for the whole scenario. The one person acting with a clear head and acting out of concern for all involved, doesn't deserve blame.