I've read that, too.
She was approx my height (5' 7"), never weighted more than 50 kilos (which means that she was dangerously underweight her whole adult life) and her waist was (with the help of a corset, for sure) at it's tiniest at 16", usually at around 18"/19" *).
That's Scarlett O'Hara territory ... and Sisi had 4 (!) children.
I was down to 49 kilo once in my life, when I was very ill, and couldn't imagine for the life of me to be in that state for 40+ years ...
*) why is that known? Apparently, she had her hairdresser keeping books on her measures on a daily basis - weight, sizes of bust, waist, hip, thigh circumference ... really obsessed with her body. And her hair which was almost floor length at one time. Took a day to wash and dry that mass.
It's said that during the hours when her hair was done, she learnt languages. She was obviously, amongst others, fluent in Greek - that good that she even translated Shakespeare dramas to Greek.
She's an interesting object to google on. No wonder she became a myth.
It's a pity that she didn't allow to be photographed in her later years (for vanity reasons, as it's said, no one should see her beauty vanishing) - so there's also almost no photos of her jewellery, too, although photography was becoming very "en vogue" in late 19th century.