He’s still useful to the people who can conjure money into being. Surely they can spare him a few oil-company board positions, book deals, newspaper columns or “consultancy” roles in think tanks, or otherwise throw him enough of an allowance to reward his service and keep him ready for the future. And then there’s the public speaking circuit.
Boy’s not going to be bagging groceries.
It’s ingrained to avoid attraction to siblings we grow up alongside, though anecdotally there have been many cases of siblings who grew up apart discovering they were siblings after meeting as strangers and feeling mutual attraction, so the biological instinct may just have evolved around the most common case.