Its a long shot, but im trying to find a story, I cant remember if its several chapters or one really long story.
A brother and sister go to the family lakehouse with their partners, the sister's boyfriend is a total tool, and the brother's girlfriend is the sister's best friend that she convinced (manipulated) into dating her brother. Slowly over a few nights, they swap partners and things gradually get more and more steamy. Finally The brother and sister have sex and the sister confesses her love, but the brother also loves the other girl, so once they return to the real world they enter into a poly relationship.
A brother and sister go to the family lakehouse with their partners, the sister's boyfriend is a total tool, and the brother's girlfriend is the sister's best friend that she convinced (manipulated) into dating her brother. Slowly over a few nights, they swap partners and things gradually get more and more steamy. Finally The brother and sister have sex and the sister confesses her love, but the brother also loves the other girl, so once they return to the real world they enter into a poly relationship.