sibs with benefits
A consensual incestuous relationship based on convenience and mutual need rather than romance. Like friends with benefits, only the couple are siblings.
Kelly was tired of having only losers for boyfriends, but she didn't want to be lonely anymore. She asked her cute little brother Eric if he was interested in adding sex to their already close relationship, and he agreed. They weren't in love or anything, it was more of a sibs with benefits arrangement.