I'm sorry to hear about your sister. I know this is a very stressful time for her and you all.
It's not unusual for lupus to present itself after a surgery. Somehow, the immune system gets off course and is thrown into a tizzy, causing it to all of a sudden turn on itself and start battling not only the bad things in the body, but the good things as well. That is what causes all the inflammation and problems.
A bad infection can also make the immune system turn on itself. Did she have an infection post-operation?
What is unusual is how quickly she got a diagnosis and how fast she started to have kidney issues. It usually takes time (many years) to acquire so many symptoms and organ involvement. Although, each person is different.
I would encourage her to join the group here too. She can learn a lot and not feel so alone in what she's going through.
Blessings,
Ginny