Familiarity, as much as it can be a vehicle for sexual attraction, can be a sexual repellent in a larger dose.
Familiarity could be the best clue available, even in a pure family. In any case, the whole purpose of sex in living things is to enhance genetic variety, so aversion to inbreeding or mating with the most familiar faces (at least in a typical tribal setting) seems reasonable.
purpose of sex in living things is to enhance genetic variety
That is simply untrue, there is not a single class in biology or anthropology that teaches such nonsense. The outcome of reproduction is a BALANCE between outbreeding and inbreeding, too much of either can be the death of a population.
I found some evidence to support my genetic variety statement. However, it seems that biologists generally say that the purpose(s) of sex aren't crystal clear, yet.
Surprise! This doesn't mean that variety has been shown to not be an important purpose for sex, but it means that I spoke too soon. Thank you T A A.