A brother or sister with whom one has incestuous relations.
Travis: Did you see Bobby Sue and Billy Ray lookin' at each other like that? It was really weird.
Clint: P'shaw, don't you know, they're Siblings with Benefits! Stay away from their trailer when their mama's workin' at the Tyson factory on Saturday nights, I tell you.
by ClimbingTheLog June 12, 2008
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