This may be my #1 read of the year, and I've read over 80 books. 
Super fast synopsis: The story focuses on Jacob and Chloe, who are twins, and their next door neighbor and BFF Leah.  Is takes place beginning their senior year of high school and fast forwards 8 years. 

This book includes so many elements: humor, slow build (x 2 because it is second chance romance), spice, sweet romance, intrigue, mystery, devastating event, emotional growth, reflection on the grief process, world travel, sports, coming of age, found family (or the family you choose), the ups and downs of friendship/relationships... This book is so well rounded.  
The character development is beautiful and Hailey Dickert did an expert job of depicting the emotional challenges of characters without excessively dwelling on the traumatic event, recovery and grief processes for too long.  To be able to fully encapsulate the characters personalities, struggles, and growth while still maintaining a good pace to the book was very impressive.  
I could not put it down. I read it in one day of travel. 
I cried tears of sadness and by the end cried tears of joy.  I hate crying when I read, lol, but this was cathartic and leaves you feeling at peace. 

Hailey Dickert is an independently published author and is one of the most well written indie authors I have read to date.  Indie authors don't have the same amount of resources and funding as traditionally published, so I usually notice a few more errors in formatting, sentence structure, or difficulty always knowing who is speaking or having a smooth flow to the story.  You would never know Hailey didn't have a slew of editors and publishers helping her out.  The Sister Between Us is SO WELL WRITTEN.  GAH. I am so so impressed. 

Favorite Quotes:
"It's not abouve how much time you have onn this earth. Time is intangible - it's irrelevant. It's about what you do with the time you have. Don't waste a second of it."

"It's hard to move forward from the worst parts of your past when you're dating someone who shares them so deeply. I suppose it's good we understand each other, but we're both in such dark places ,and like Martin Luther King Jr. said, "Darkness cannot drive out darkness; only light can do that."

"Honestly, is there anything sexier than a man who reads?"

"Ti penso ognni giorno."  and "Sei il grande amore della mia vita."

"When we got home, I took a shower and shaved everywhere. I'm talking HAIRLESS AS A DOLPHIN."  bwahahahahahaha. That cracked me up more than it probably should have.

Favorite Pages (because the amount of sentences I loved were too long, lol):
Page 69 - swoon
Page 196 - ugh, lovebugs
Page 244 - insert ugly cry because it is so deep

2-3 chili peppers for spice. There are primarily 3 spicy scene and Hailey has a Dicktionary in the back of the book so you can either go straight to them or skip over them, lol. Very creative. OH and she has a playlist in the book!