Don’t ask me the how or why, I suspect that since she moved in they’ve started to do things a mother and son should not do. I’m not sure what I should be doing because if I’m right and turn them in they could be arrested. They’re good people just doing something screwed up and no one is getting hurt. At the same time I can’t help but think this is wrong. Am I right to tell them they need help, or should I walk away and leave them alone?
Yes please, tell them they need help. You need to clear your conscience of all guilt hence the need to let them you think what is going on between them sexually is not normal.
But don’t in any case report to any authority at all – Like you said, they are condescending adults who should know their right from left (not necessarily wrong).