] Although, as they play no part in the story, she might have chosen not to mention them at all. There really is no evidence one way or the other. It is all speculation.

... and JA doesn't mention any, then I conclude there are none. It is easier for me to assume no other Bingley siblings than to assume any number of unmentioned siblings. If there were younger sisters to look after, then JA could have used them as an opportunity to draw a comparison between Bingley's responsibility for them and Darcy's responsibility for Georgiana. I also find it hard to believe that Caroline & Louisa would never mention any younger sisters, even if they were quite a bit younger.