>there is some part of the human brain that suppresses sexual desire towards those it codes as "family members."
The other condition of this idea might be because many of us are inherently disgusted by the things our family does.
For example, if you have that friend with the really hot sister: "Dude, your sister is really hot!" Now, his initial reaction to you is going to be, "No way, man!" partly out of embarassment, partly because of social taboo.
But if you were that guy with the hot sister, chances are that you might find her attractive if she weren't in your family. This is because the family sees all the more intimidate sides of their personalities. She could have a ton of disgusting habits/traits/characteristics that she doesn't reveal to anyone but her family. Or it could be like "cabin fever"; being around her so much causes you to hate said traits even if you didn't hate them immediately.
It is for this reason that your quotation may not only be a brain mechanism but rather conditioning toward your society(?) I'm not sure if I worded the last part correctly.