I think I've come to an idea which could work with this start, although I don't know if the mangaka is planning that (or even any other continuation)...
...but let's say for a moment that the opening lines aren't figurative at all.
The 'sister' is a literal supernatural creature, and the opening line is a massive mislead. Maybe she/they never call themselves a demon, maybe they do, but as the story goes on, it's made increasingly clear that they have powers -- including the ability to cause sexual attraction in humans.
That's the only continuation I could imagine -- and nearly literal demons torturing a mother and daughter would probably allow the needed escalation from the starting point.