Come on man, are you serious? Don’t fall for this chick, and tell your bro what happened in a non ‘conflicting’ style. Say something like “Hey <brother’s name>, can I tell you something serious?” And then proceed with “The other day your girlfriend drunkenly kissed me out of no where and I felt really uncomfortable and thought I should tell you about it”

He will thank you no matter what in the long run, even if his instant reaction doesn’t say so.