I took the opposite route from your Mother: I did nonfiction first.
I really don't know why, but everyone in my immediate family, going back several generations and forward one after me, does the fiction first. My grandson, OTOH, has always read fiction and non-fiction, about to the same extent; though now is a bit heavier in reading non-required/for his own pleasure, non-fiction books.
As long as a person is well versed in both kinds, I doubt it matters how one begins/ends up with reading choices.