We read that Sissy went on long walks, and her entourage could hardly keep up with her.
She rode at hunts all day and exhausted many of her male companions.
It seems she was obsessed with keeping her figure and keeping slim.
( a little like Lady Di )
Corsets were a quick way to get slim and Sissy seems to have been a devotee of the corset...
Perhaps back then it was a relatively new aid to female beauty..... and new
corsets ( whalebone ? ) were appearing in the shops every week !

This subject obviously needs a lot of research !