This is one of those situations where there is no right answer. My go-to response would be - if the girl showed interest in reading them, then give her one and see how she likes it. If it's too "adult" in tone - not graphic sex and violence, just adults "adulting" - then she might find it incredibly boring or dense. Be open to discussing the book with her, if she has questions.

But, since you and your sister are not this girl's parents...you're risking a big mess, sadly. Whether the girl likes the books or not, if her parents disapprove then you just broke a big family rule and you won't have any say in how they interact with their child.

If you know the parents well enough to talk with them about it, then you can say, "Look, I think she'd like these books but I can't be sure. As far as I know, there is nothing overly shocking, traumatic, upsetting, disturbing, or graphic in them. As far as I know, after doing a little research online - haven't read them myself. I think curiosity should be encouraged and I think reading is a safe way to explore and learn about the world. But you are her parents, so you know her better than I do. Also, you are the ones who will deal with the consequences - whether that means discussing topics you are uncomfortable with or your wallet takes a hit because she wants to buy all 20 books this author has written to date."