Incest Is Relative

"But that's mental! Why would any guy want to fuck his own mum?"
"Yeah. You hear about it on the news and stuff, though. Mums who fuck their sons, dads who fuck their daughters and their sons. Brothers and sisters who fuck each other—"
"Language, please!" Miss Azley protests.
"That's bullshit. Who would want to fuck—sorry, screw—their own parents?"
"It's called incest, man."
"That's when a guy rapes his sister, dickhead."
Miss Azley's English class, Forbidden

Tropes relating to incest and its various permutations.

If that's not incestuous enough, then Screw Yourself. When it's Subtext, rather than Canon, see Incest Subtext, for the shipping, see Incest Yay Shipping.
http://tvtropes.org/pmwiki/pmwiki.php/Main/IncestIsRelative