Identical Twins Got Married On Accident!

The "oops, I accidentally had sex with a family member" theme is unavoidably interesting, which is probably why it keeps echoing through literature, pop culture and trashy, unverified news pieces.

The granddaddy of them all is Oedipus Rex, of course. Hard to beat that for sheer horror. I'd like to cite a particular semi-recent film that does an awesome job exploiting this theme as well, but to name the film would be to spoil the nasty surprise, so I won't. And then there's Heinlein's "All You Zombies--" in which time travel and polymorphous sexuality creates a series of ridiculous paradoxes. The spoiler-laden Wikipedia entry is here: [link]