diet incest

Being an adopted child of no blood relation to one's adopted parents, the act of having sex with a relative. It has all the flavor of incest without the incest, technically since there is no close biological link between the two (or more) individuals partaking in the act.

Mike: Hey, Nick, did you hear Brian fucked his older sister?

Nick: Awww, gross, dude!? What the fuck?

Mike: I know, but technically they're not related. Brian is adopted; so, it was diet incest.