Yes, real brother and sister. Now, the relationship may not be exactly what you're thinking about, although there are certainly some indications in that direction. The whole dynamic between them just becomes more and more unsettling as the show goes along. I'd like to say something else, but I don't want to spoil things in case you do ever get a chance to see it (or the DVDs become available in a format you can play). All I'll say is that the sister warrants more attention than she seems to.