1. That Summer
Honestly it's the rare Dessen book that doesn't have a sister relationship in it, but I definitely have a soft spot for this one. The messy relationship and misunderstandings that exist between Haven and her older, wedding-crazed sister, are lovely and poignant to me. Other great Dessen books with sisters include Dreamland (where a sister running away is a catalyst for something much worse), The Truth About Forever (where this relationship is in the background of everything else), and Just Listen.
2. Sisterhood of the Traveling Pants (series)
I have much more than one issue with the Sisterhood of the Traveling Pants movies, but the main one is definitely that Lena's younger sister Effie wasn't a part of them. In the books, the relationship between beautiful Lena and her younger sister Effie is, if not a main plot, at least a major subplot. There's a lot there and despite being in the background for most of the time, the relationship these two share and Lena's thoughts and feelings toward her younger sister ring absolutely true and wonderful. The push-and-pull of Lena, Effie, and Lena's friends is brilliant. These two are definitely one of my favorite sets of sisters in literature.
Though not sisters in the traditional sense of the word, Rhine and her sister wives are easily my favorite part of this book. Within an incredibly strange situation that none of the girls particularly wanted to end up in (except maybe the youngest), these three form some intense and complicated friendships that mirror those of any sisters.
4. The Extraordinary Secrets of April, May, and June
If I had put this list in any kind of order, this book would most likely be up at the top. The voices, feelings, and relationships of these three sisters are authentic and at the forefront of the novel. Their dialogue is funny and sweet and just like conversations between actual sisters. I love it, I love it, I love it.
*Also, in a related note, I'll be reading Nova Ren Suma's novel Imaginary Girls soon and that is also about sisters. I'm so excited!