My parents were both hospital doctors and they absolutely did talk about patients, bollocks to that not happening (at least in the past).
They did not use names though. So it was all "a patient"
They also showed slides (this dates them) at home of surgeries more than once - I vividly remember a slide of a huge benign tumour removed from a female patient, who was shown in the next slide before and after. They were slides of the kind you'd see as illustrations in the BMJ. I have no idea what on earth they thought they were doing showing these slides to their early and pre teen children, except they were still labouring under the illusion that like a lot of doctor's children we'd grow up aspiring to go to medical school to follow in their footsteps.
People like to think doctors have super human levels of professionalism and discretion, and don't have time to gossip/ chat, but of course they do socially!
Doctors are just people, there's nothing special about them, they just took medicine at uni not law or engineering...
As I said, the one thing they didn't do is use names, so your sister is fine unless the surgery is unusual enough to identify her, I suppose.
Why is she worried about this? Does she think she was disloyal to change doctors or something? Otherwise it would seem not to matter particularly.