I think when someone demonstrates certain behavioral patterns it’s best to believe those patterns and not have expectations otherwise.
It’s probably time to stop expecting her to respond in a courteous normal manner or have respect for anyone’s feelings other than her own. The little one won’t be upset anymore if they are not put into that situation again. Your sister won’t have to sit and wait it out for 3 hours anymore either.
Alcohol is a progressive disease and as her disease progresses so will her negative behaviors.