Could someone attempt to explain something to me?

Yes, they are brother and sister. I think their relationship was just showing another side, a darker side. What i got from this movie is that nothing is as it seems on the surface with the a lot of these characters. I think the Markus Ori relationship was there to show that even though Ori is seemingly the only "happy" student in school..her life really isn't any better than anyone elses. She's dealing with a whole set of messed up issues, her sexually relationship with her brother and the fact that her parents don't want their daughter living with them. I think it's there to prove that none of these teenager's lives were perfect, they were all going through a lot of traumatic things.