I am reminded of the recent case in which twins, separated and adopted at birth, later met and married.
In that case the argument was that their shared genes predisposed them to be attracted to each other. Presumably, had they grown up together, the normal" taboos would have been operative.
The Boy has a younger sister (
) he has never met and is presently unaware of. She is, I gather, much like him in temperament (
).
One to watch