I tell her I’m not interested (unless she’s so drunk that it might not be indicative of her ual interest) and move on, gently persuading her in the direction of my brother the rest of the evening and then I tell my brother a day or two later about the kiss that happened. I tell him I’m not in love with her and not trying to pursue her and then let’s him know that if he needs help dealing with the emotions surrounding it, we can talk it out. If he needs to be left alone then I’m willing to leave him alone too.
And then I make sure to continue showing that disinterest in her so nobody gets mixed signals.

Omo.....that's quite the incident. 죄송합니다! If this is an actual thing that happened, be sure that I am PRAYING FOR YOU. I guess if I was in that situation, I would ask my brothers first love why on this flipping earth she did that, and the rest would depend on what her answer was. If she said that she had been in love with me the whole time, I would ask her to let me think for a while, and I would try to find the best way to let my bro know. If she said she did it under the heat of the moment, and it was nothing more than a mistake, I would say "Well, it was really weird of you to do that, and you could've potentially broken my poor bro's heart, but I am willing to forget about it if you are." Then again....everything would be different if I actually had feelings for this "first love" of my brothers. It could really go any way.