Psychological stress could be a more current version of what was bandied about considerably in psychiatric and psychology circles as "psychosomatic." Freud pretty much coined that. I wouldn't dismiss it. And Jr's sister having a case doesn't disprove psychosomatic origins. It strengthens the genetic argument but doesn't guarantee it at all. In any case, when I hear him I wish I didn't even before I manage to make any sense of his utterances simply because the very sound of his voice is offensive.
I absolutely dismiss the notion of 'psychosomatic'. They keep changing the terminology to make it sound more science-y (from hysteria and the 'wandering womb' to conversion disorder or somatisation disorder, to 'functional neurological disorder') but it's the same thing in the end - demonic possession. The invisible demons doing their demonic work, conveniently explaining away anything doctors can't currently understand, thus doing the important work of protecting the medical ego.