I've genuinely never heard that in my life.
Not wanting to fuck your sister really has nothing to do with whether or not you're related.
IIRC the psychological block happens when you grow up around the person....since you typically grow up around your relatives, you just naturally learn to not be attracted to people who look like them.
Outside of that, I think people are naturally attracted to people who favor them. So yeah, if the first time you ever meet your relative is when they're an adult,
and you are oblivious to the fact that they're related to you....yeah, sexual attraction is a thing
This means absolutely nothing in context of Jamie and Cersei though, which is why it's gross
It absolutely applies to Jon and Dany though