Books have a much higher production value than content farm material like your sister writes. They also rely heavily on reviews, which is incentive to invest the effort to do it right.
Modern recipe books have photos of the dish, so obviously they made it at least once. It would be unreasonable for them to invest all of the effort into writing a book, typesetting it, creating photography to match, proofreading, and getting it published, but to not actually try the recipe inside.
I’m sorry, but your sister is projecting her behavior on to everyone else in order to justify it. She is writing content farm material, but that doesn’t mean the entire industry works that way. Especially not the paid material book industry that hinges on reviews, gifting, and referrals.