OK, two more for you. One of the stories in
Swan LakeSpoiler (highlight to view)
is about a girl who's in a relationship with her cousin (which seems to be acceptable), but she discovers that he's actually her half brother. The girl was born from her mother's affair with the mother's sister's husband, so you've got double incest, yay~
I don't know if you want to get into novels, but I just finished
Kafka on the Shore by Murakami Haruki, in which
Spoiler (highlight to view)
the 15-year-old protagonist has a sexual relationship with the woman he believes to be the mother who abandoned him when he was four years old. She knows he thinks he's her son, and she would know if it was true or not... Another character also knows about his suspicions and his affair, and it's no big deal. I think that she actually is his mother, though it's never 100% clearly stated. (He also seems to be some kind of reincarnation of her first love (who is not his father), the whole thing is interesting but quite confusing.)
This is a mainstream novel, btw, a "national bestseller" according to the cover. *I* found it rather disturbing, but none of the characters in the book seemed to be too bothered by it!
________________
"[English] not only borrows words from other languages; it has on occasion chased other languages down dark alley-ways, clubbed them unconscious and rifled their pockets for new vocabulary."
-James Nicoll, can.general, March 21, 1992