Say You'll Be Mine is book #4 of the Stargazing series, a 12-book series of standalones.

I don't remember the moment I fell in love with Edith Kennedy, but I remember the day I stopped loving her—or at least the day I told myself I'd stopped.
She is a breath of fresh air. She makes me smile; she makes me whole.
But when the unthinkable happens in my marriage, the love I had suppressed for her comes back full force.
She is my best friend—the one I’ve always loved.
She is my wife's sister.

 

I fell in love with my best friend, Joel Fitzpatrick, when I was fifteen.
With a single touch—a single, spine-tingling kiss—I knew he was the one, even if my love was unrequited.
As the years pass, I find solace in our friendship when my life is turned upside down.
But when a seemingly innocent night leaves me riddled with guilt, the life I’d dared to imagine seems nothing more than impossible.
He is my best friend—the one my heart belongs to.
He will never be mine.