Boyfriend doesn't remember last night....?

If you are absolutely certain that was his sister he might deserve another chance. As long as that kind of binge drinking doesn't become a habit. If that happens - run for the hills. He should be held responsible in the sense that he knows you won't tolerate it ever again. Every one deserves a second chance - as long as you're sure that was his sister? That sounds so weird to me - unless his sister doesn't like you for some reason.