it's just incest.... shimmer the hell down. it was just a crush from the family sped, not her own dead parents coming back to take her for a test drive.

and stepping away from your own biases lets look at the geo/sociological structure of spira... sins been slaughtering people for "a thousand years" so that judging from various cutscenes world population is reduced to about 1 million creating a shrunken gene pool, incest becomes a unavoidable consequence/happy occurrence.