Hi!
According to Angela Duckworth, a psychologist, in an investigation from 2016, the more important factor that can predict success is a mixture of passion and persistence she calls "grit".
She says that "gritty" people achieve more success than anyone else, for example, she found that grit was the best predictor of good performance in Scripps National Spelling Bee contest.
The conclusion of her research is that success is predicted by grit, so your sister is right, having grit is a better predictor of success in school.
