Waking Up in Vegas by Natasha Preston

There are bad hangovers, then there’s waking up married to your brother’s idiot friend.

On the trip of a lifetime, a drunken Wren and intoxicated Brody stumble upon a wedding ceremony and decide to tag along for the ride. But being spectators isn’t quite wild enough when you’ve got tequila in your toes, and before either of them can stop what they’re about to do, “We now pronounce you husband and wife.”

Once sober and home, the incompatible newlyweds try everything within their power to end their pantomime marriage. Unfortunately for them, the promise and vows they made to spend their life together now aren’t as easy to break as they’d once hoped.

Especially not when Brody starts to have a change of heart.