Feast by Hannah Murray

Poor Spence—all he wanted to do was fly to Vegas, walk his mother down the aisle, and get back to his shop. Until a very attractive seatmate on the plane has him adjusting his plans to make time for some anonymous stranger sex.
It’s just his bad luck that his anonymous stranger turns out to be his new stepsister.
When Maddie realizes she’s just spent two days banging her new stepbrother, her first reaction is to laugh. Her second is to wish she could do it again. It was very memorable banging, and the prospect of not doing it anymore is a bleak one. Still, she can keep things sibling-like if Spence can.
Lucky for her, Spence is just as eager as she is to pick up where they left off in Vegas, on the condition they keep it just between them. Telling the family will only make things complicated, after all. And that’s fine with Maddie—until she realizes she’s falling in love with a man who seems content for her to stay his dirty little secret.
Dirty she’s fine with. A secret? That’s a deal breaker.