It Starts with Us by Colleen Hoover

Allysa’s eyes are swimming in puzzlement. “I still don’t get it. If you
weren’t actively cheating on him with the guy, why does Ryle hate him?”
I release an exaggerated sigh. “I’ve asked myself that a million times,
Allysa.”
She makes an annoyed face only siblings could reserve for each other. “I
never wanted to ask because I thought you were ashamed that you cheated on
my brother and just didn’t want to tell me.”
“I haven’t even kissed Atlas since I was sixteen. Ryle just couldn’t handle
that my past sometimes crept into my present, in an absolutely platonic way.”
“Wait. You haven’t kissed Atlas since you were sixteen?” She latched on
to the absolute wrong point of this conversation. “Not even on your date last
week?”
“We’re taking it slow. And that’s fine by me. The slower we take things,
the more time it gives me before I have to break it to your brother.”
“I think you should just rip off the Band-Aid.” She points at my phone on
the counter. “Text Ryle right now and tell him you’re dating Atlas. He’ll get
over it; he doesn’t have a choice.”
“This is something I need to tell him in person.”
“You’re too considerate.”
“You’re too naïve. If you think Ryle is going to get over it, you don’t
know your brother very well.”
“I’ve never claimed to.” Allysa sighs and drops her chin into her hand.
“Marshall told me he told you I cheated on him.”
I am so glad she’s changing the subject. “Yeah, that was a shock.”
“Drunken mistake. I was nineteen; nothing counts before you turn twentyone.”
I laugh. “Is that right?”
“Yep.” She hops on the counter and starts swinging her legs. “Tell me
more about Atlas. Tell me like I’m your best friend and not your exhusband’s
sister.”
And we’re back to this conversation. That was a quick break. “You sure
this isn’t awkward for you?”
“Why, because Ryle is my brother? No, not awkward at all. He should
have been nicer to you, and then you wouldn’t have to date Greek gods.” She
wiggles her eyebrows with a grin. “So, what’s he like? He seems
mysterious.”