In a household of 5 sisters and no brothers, the girls were probably used to flying around between their bedrooms half dressed. I think the little scene was added for fun and to give us an idea of the upstairs domestic arrangements of the Bennet girls. I'm unsure what to make of his reaction. Was that a bible he put up to his face? You don't suppose he was warding off sudden temptation ? Then regaining his composure he soberly walked down the stairs. So typical Lydia would forget their houseguest. LOL!