Quiet Ones (Hellbent, #3) by Penelope Douglas

Quinn
I've always had too many men in my life. My father, brothers, and nephews. They run this town, and although I've never given them a reason to worry, they treat me like I'm made of glass.
They're wrong, though. I've got it all together - college graduated by twenty-one and a business owner with a perfect record.
But I have one secret: I raced to grow up in case he ever came home.
Lucas Morrow grew up with us, but he left when I was thirteen. I've waited and worked to be sure everything is perfect when I see him again.
But I've waited years and maybe it's time to move on, get my own place and be my own person with new friends.
It's summer after all and I am my brothers' sister. Did they really think the apple fell that far?

Lucas
She thinks her family are the only men watching her. So innocent.
And I want to keep her that way. Even though I'm twelve years older, we were always the odd ones out.. She gravitated to me, and I always made sure she wasn't alone. When I left, I told myself she'd be fine without me.
But really, it was just an excuse to escape the guilt.
I only meant to come back for a few days, but she's grown up, her brothers are distracted, and one bad decision after another is circling her.
As well as old ghosts.
And old enemies.
She doesn't want a babysitter anymore, and if she ever finds out why I left town, she'll know why I'm not the best role model.
But I'm still the older one.
And she still needs supervision.