Unveiling Hope by Jeannine Allison

Samantha is aimless. A college dropout who’s just been kicked out of her house, she’s searching for herself. But she never thought she’d find herself moving into the apartment of the man she’s spent the last year trying to forget.
Derek has secrets. An underground fighter who acts first and thinks later, he’s determined to keep that part of his life hidden. But when he’s forced to offer Sam—the girl he can’t stop thinking about—a place to live, his carefully constructed lies unravel.
Everything is stacked against them. Yet neither can walk away.
She’s seven years younger.
She’s his best friend’s little sister.
She has a boyfriend.
There are lines you don’t cross… and they’re about to obliterate them.