In the addled brain of a perv, he sometimes thinks women want what he is doing - even if they don’t say it out loud. He may have seen her as being sexual because he obviously sees even young girls as sexual objects. Perhaps he conveyed to his son that the wife was a floozie, or maybe both men are just nuts! It still defies logic that the father would admit on camera that he was banging his daughter in law.
'Father-in-law, daughter-in-law flirting with each other, maybe some sexual touching or whatever. And I enjoyed it, frankly. '
'We interacted in a lot of sexual ways because Susan enjoys doing that.'
From: Police-search-home-husband-father-law-missing-Utah-mother.
He read his daughter-in-law's teenage diaries. He ENJOYED what he called sexual flirtation (and he was the one who developed fantasies around it).
What is not explained very well by the FIL is why the Daughter in law moved to UTAH to get away from him.