Were the mother and son sleeping together?? At first I thought it had to be part of the charade as humans, her just pretending to be his mom, then I thought maybe she created him in a vampire way that was different from a human way. This sustained me till the end of the film.
Well for me, I always thought that the Sleepwalkers having intercourse was the way they fed each other. If you look, when they are in the act, an unmotivated light is actually forming beneath them as if they are transferring energy between the two of them. Then later in that scene, we can see that the energy they generate (even from not having eaten for a while) is great from looking at the window of the room that they are in. It actually manifests itself in an odd blue light and due to our culture, (which I'm sure is different from theirs) we interpret that as intercourse. I mean in vampire lore, Vampires do not engage in the act and merely use it as a tool to lure victims for themselves to feed on.