Two people who happen to be dating and also happen to look like they are siblings; generally greeted with fear and disgust. Normally they are made fun of by people who have no idea who they are but see them 'macking' on each other in public.
Extra Points: They dress similarly. They have similar names. They randomly have the same last name and/or they are actually siblings.
"In the long run it's a good thing you and Fredrick didn't go out, because you would be one of those gross sibling couples."