The automatic comparison that comes to mind is Sweet Valley Twins. Sammie (Samantha) and Charlie (Charlotte) are identical twins who are starting 7th grade at a new middle school. Thanks to their father's job at a sports club, they've managed to befriend the popular pre-teens whose parents frequent the club. Charlie, who is thinner and more interested in fashion and makeup, fits in better with the group, whereas Sammie struggles with this group of people who don't really "get" her and also bore her.
They've also competitive tennis players for most of their lives, coached by their dad. Though both Sammie and Charlie excel at tennis, this book is the start of the two girls growing up and finding their own interests and friends.
This book is for middle-grade readers, and Sammie's voice (book #1 is told from Sammie's POV) seems pretty authentic. The book stays pretty superficial. Though we dig down into Sammie's emotions about how her path and her sister's are diverging, and how she feels to be left behind or left out (#relatable), that's the most serious any problem ever gets. For that reason, I tagged it as fluff. Sammie is also surrounded by nurturing adults like her grandma and English teacher/club leader, who support her always and show her how to accept herself and the changes that are happening in her life. There were times during the twins' conflicts when I felt Sammie's parents and grandma were heavy-handed with lectures, but I suppose they didn't even get real lectures...just encouragement to apologize and reconcile. (Interesting how you never see that in YA books...)
Though I felt the main characters could have been more fully developed and not so thinly sketched, I guess that will come with time, as this book is part of a series. I liked the assortment of secondary characters that we're introduced to, and I also liked how the popular kids weren't completely one-dimensional, either. It seemed like some of them were pretty nice, and though they could be judgy and exclusionary, they weren't dirtbags or bullies (except for the rude comments one of the boys made about Sammie's weight). They were more like kids who expressed strong opinions about what they liked or thought was cool, and those things just happened to fit a narrow mold.
I also liked how open the characters were when they communicated. Instead of letting things that should be said fester in silence and growing angry or neurotic over them, Sammie, her twin, and the lead popular girl Lauren were forthright whenever they had an issue or thought. I was pretty amazed by Sammie standing up for herself multiple times -- I was too meek to do that as a 7th grader, even as an adult sometimes -- but to balance this tough girl side out, there were plenty of times when she was hurt or felt betrayed by her sister's actions and just needed to cry it out or talk to someone.
I was also glad that Sammie had her grandmother GoGo to confide in, and that though her mom was away at culinary school in Boston, her mom was supportive as well. Sammie and Charlie also have an obnoxious older brother who's an 8th grader, but he's there to support them when it really counts.
It was also a treat to discover that the book is set in West LA, where I currently live (I assume that's how I found this book at Goodwill). Most of the places namedropped in the book, like Third Street Promenade in Santa Monica, various beaches and street names, were excitingly familiar.