Incestophilia

Children show erotic ties to parents and brothers and sisters, though not before 4–6 years of age. Infantile incestophilia is manifested by sensations and phantasies; incestuous acts, even sexual acts, may occur among brothers and sisters.

For example, there seems to actually be a disinclination to mate with a near relative (given alternatives) (and yes, various forms of incestophilia abound, but still). But it also seems not to be based on any perception of relatedness, at least not in humans, but rather on whether you were brought up together.