I've been thinking of a few of my favourite books that are either 'love them or hate them' types.
Important Artifacts and Personal Property from the Collection of Lenore Doolan and Harold Morris: Including Books, Street Fashion and Jewelry by Leanne Shapton.
This one is set out like an auction catalogue and tells the story of a four year relationship, with the property of the couple being auctioned after they split up. It's an odd one, not for everyone, but if your sister is up for something a little bit different then this may be the book.
The End of Mr Y by Scarlett Thomas.
Ariel finds a rare book which is also rumoured to be cursed and (as it says on the amazon blurb) finds herself in an adventure of love, sex, death, and time travel. She also meets the god of mice. It's very difficult to describe this book and do it justice, but again it's an unusual one that your sister will either love or hate.
A slightly less hard going but still unusual one is Our Tragic Universe, also by Scarlett Thomas, which is my favourite of her books.
Antichrista by Amelie Nothomb
Is about a teenage girl named Blanche, who makes friends with Christa on her first day of college, but she soon learns that friends can make the worst enemies. It's not something I've found on sale easily here, amazon has copies.
Where were you Robert? by Hans Magnus Enzensberger
A teenage boy disappears through his TV screen and travels back through time, searching for a way to return to his home in the present.
The Ritual by Adam Nevill
If you think she might like something a bit scary, this might be the book she'd like but hasn't heard of. Four men are lost hiking in the ancient forests of Sweden, and become aware of something stalking them through the woods, picking them off one by one.
A Kind of Intimacy by Jenn Ashworth
Annie moves to a new home, and becomes interested in her new neighbours, with terrible consequences. There's a slowly building awfulness in this one.
The House Next Door by Anne Rivers Siddons
Again, there's a slowly building awfulness (and some horrible, very well written scenes) and Colquit and Walter become aware that the newly build house next door to theirs hides a kind of evil that terrorises their neighbourhood.