There's a big difference between reacting to a threat placed on you in a decisive way and travelling somewhere to create the encounter yourself.
I have a younger sister, and in my school days spent a lot of time 'protecting her' so I understand the emotion, but you can't be going hunting to hand out punishment beatings mate. I'm pretty sure that's not what Richie has in mind with his material. Self protection and vigilantism are two different things.
If you really think you're at risk of a group attack now as a result of last night's actions you may have to heighten your awareness and practice a little sensible avoidance for a while. Alternatively, you could contact the guys in the cold light of day and calmly explain you didn't appreciate his behaviour and would prefer him to leave your sister alone in future. That might defuse the situation before it escalates.
Incidentally, you can taste rum in coke unless it's in quantities too small to make a difference. I'm not saying the guys weren't at fault, but I am saying it's unlikely your sister didn't know she was drinking. They haven't taken advantage of the situation, which you can be glad of, but they could of done. Make sure SHE learns from the event. That's the bet way you can protect her.
Good luck