A married woman had more freedom in society than an unmarried one. And I think a married sister alone could chaperone a single sister to a small party. Probably to attend a larger social event, a girl's sister and brother-in-law may be required. I could be wobbly on the latter point. Hmmm. I'm just trying to think what Miss Bingley would do. I'm unsure if she and Mrs Hurst could attend a ball without at least Charles or dragging Mr Hurst off the velvet couch for a male escort.