My Rival Valentine by Jessica Crafts

Let the games begin.
Alexa and Sean are best friends, former step-sibs, and remorseless heartbreakers. After yet another kiss-and-tell session about their latest conquests, they both realize they’re bored and that they’ve nailed every desirable single in the city. So Alexa proposes a bet.

The first one to get someone to say “I love you” to them before Valentine’s Day wins.
They choose the targets for each other and with only two weeks to seduce their prey, the game is on. Alexa has done her best to set Sean up to fail by choosing the one woman least likely to fall in love with him. She has her eye on the prize – Sean’s house in the Hamptons – because Sean has it all… All, that is, except Alexa. And he’s decided the term of the bet is if he wins, he wins her.

But there’s more at stake than either of them realize.
It doesn’t take long before Alexa gets sick of seeing Sean try to woo another woman. Which is exactly what Sean wants, because as soon as Alexa’s heart is no longer in the game, Sean will claim his prize. He’ll claim her and, unlike the experiences that bonded them as teenagers, he won’t let her go this time.