To me, this illustrates a possible consequence of "putting the old folks away" when they start to lose their wits. Because their brains aren't reactive like they were, some relatives stop thinking of them as people--just hollowed out shells of who they used to be. And "putting them away" becomes a "just make them comfortable" sort of guardianship, where most forms of mental stimulation are muted and all life activities are highly regimented.
These two found an incredible way to share both physical and mental stimulation, and their drives for getting into it are obviously strong. I say set them up in the honeymoon suite, give them a mirror on the ceiling, trapeze bars, anything they want, including a regular look-in by a doctor to make sure they are not hurting each other.
I also say get a hooker to untwist the son, too. Mouth on penis--the horror!
Let me guess. The son will probably file suit--one of those righteous indignation paydays.