^ Nursing student here! As I understand it, long-term kidney damage/nephron death is irreversible, but short-term impairment due to inflammation, especially as a consequence of infection, will subside with treatment of the underlying problem. So kidney function
can certainly improve in the short term, and overall management of chronic kidney disease can ward off acute "kidney attacks," if you will, but that doesn't mean the kidneys themselves
have improved, in the sense that their optimal functioning potential cannot recover to previous baselines. Specifically, kidney function is measured via the glomerular filtration rate, which will improve when nephrons, the functional units of the kidney, are not actively inflamed. (Determining the exact extent of nephron death within a kidney is a difficult and inexact science without extracting it and cutting it open, which is obviously impractical in a living patient.) Science!
Anyhow, glad your mother's doing better.
As for your sister, that's a tough situation. My sympathies, and my advice would be to try not to get upset and lose your cool. I'm sure it's hard, and I don't mean to be flippant, but raising ire on all sides will likely inspire defiance, rather than cooperation, in your patie- er, sibling.