I don’t think incest is giggled about, it’s a quick way of setting the characters as off. Notice in most of their examples at least one and sometimes both characters are sociopaths or serial killers, not exactly a ringing endorsement for keeping it in the family.
“... it’s a quick way of setting the characters as off”.
I agree. I remember reading, “The Fall of the House of Usher” (Edgar Allan Poe) and there was incest themes. By themes.. I mean it was suggested. This was definitely a way to set the characters off as not normal.