I can answer that with one word: Taboo, taboo is probably the number cause of most fetishes out there, its something your not suppose to enjoy, and that just makes you want it more.
Eh, I dunno if that explains why sibling relationships were not immediately disgusting compared to parent-child relationships.
I chalk it up to it being in a realm of stories completely removed from our time, y'know? Cuz when incest crops up now you're like "ewwwwwwwwwwwwwwwwwwwww" but when it crops up in old stories about Greek Gods you just kinda shrug and go "eh, probably not the weirdest things they've done."
Man the stories I would like to hear from this.....
Never ever let someone tell you the Old Testament is not full of fun stories. Like f'reels. Those crazy plagues are only the tip of the iceberg.