Has your sister read Wings of Fire? Those stories and that content deal with heavy topics that aren't just for kids and CERTAINLY take a mature mind to analyze in certain situations. I started reading the series in elementary, and I'm an adult now; I know people nearing their 30s that love Wings of Fire. Don't be ashamed because people call it a kids' book. Sure, a 3rd grader should be able to read it. But it takes someone who's seen a lot and knows a good bit to actually analyze it.
Sincerely,
An unabashedly die-hard Wings of Fire fan