Have the Bennets ever experienced a family disaster before? Both Mr and Mrs Bennet are alive, and there's no mention of the girls having a sister or brother who didn't survive childhood, so it's quite possible that there's been no death in the immediate family. There's no mention of any grandparents to Elizabeth and her sisters, so quite possibly the grandparents died before the girls were born or when they were too young to realise what was going on. Mr Bennet has kept the family out of any immediate financial trouble (chpt 50). It's quite possible that this is the first family disaster to really affect the Bennets, so Elizabeth may never have seen before how Mary reacts to that sort of stress and uproar.