[clitty]

People who have a sexually or emotionally sexually relationship with show it in their body language... even very small things.

Hence, office affairs are noticeable by other employees. There is flirting, but things go up a notch when they are fucking - either they touch more or they kind-of avoid each other to appear normal.

sibling sex is common - especially with twins, but still icky. parent<>child (adult) is very icky. But that usually means that sort of thing started out when the child was underage and has also caused some mental issues that carries on into adulthood.