Apple Crush by Lucy Knisley

Jen is just getting used to her life on Peapod Farm with her brand-new step-sisters, Andy and Reese. But when the school year starts, there are even more changes in store for her. Jen has to navigate new friends and new challenges--but at least she'll have Andy with her, right? As school begins, she finds that her step-sister seems way more interested in crushes and boys than hanging out with her, while Jen wants to know when the world decided boys and girls couldn't be just friends anymore.