Incest
In real life incest is an abhorant act but in the context of this movie it seems resonable with the parties invovled. The mother and son sleepwalkers are the last of their kind. If they don't have sex with each other then who are they supposed to have sex with? Maybe they can't procreate with humans. Now the mother is probably passed the point to conceive another sleepwalker but they being the last of their kind they probably developed an attraction to one another on an instintual level.
You can't think of these people as humans. That is why in their sex scene the camera pans over to the mirror and we see them in their true form.
And who knows, maybe in the sleeperwalker world it's okay to sleep with someone in your immediate famly.