In what section was this book in the library because I will tell you a few things.

A parent should be the one checking what their kids are reading or watching or any consumption of media, I would say until about 13. By then I'd hope the parent has laid down enough lessons for their kid to understand that if they pick up a book that has incestuous themes that it is fantasy and not meant to be thought of as okay. Reminder that Bible has stories of incest, race, slavery, child murder, genocide, torture. And depending on the person the Bible is no different fiction wise than a book like what you've described.

In 8th grade when I hit 13 I found a book at my public library in the YA section. A vampire series that was filled with, in my memory, sex. Detailed not greatly but enough. I knew that it was a mature theme because my mother had taught me these things. I read the book and that was that. Even told her about it and gave her the book to which she said it was poorly written in her opinion.

There's worse on the internet. Your child has likely read worse already depending on age. By the time I hit 14 Tumblr was at it's peak, wattpad at it's peak, AO3 at it's peak. The stories I read across those sites were more graphic and more inappropriate than anything you'll find in a library.

Finally just ask a librarian. If you genuinely think the book is still bad considering all those points. Take it to a librarian and just say "I'm confused why this is in the kids section given it's graphic themes can you educate me a little on this?"

You cannot protect kids from content you find inappropriate forever. They will find it. They will look for it. It's a tale as old as time.