Supposedly, and according to the previews, Takeru will transform again in eps 7.

Based on the intro to the first eps where she says all that stuff about mass extinction, I've thought that Hikari was the destroyer of species. This seems to be backed up by Takeru's transformed shape (the whole brutality thing) and that she completely obliterated that monster. Not to mention the creppy things she says when she gets all 'evil.' If they are like sisters, then Akari must be the creator of new species after everything is all dead. This because of the weird species she had around her in her introductory eps, the bettle (not the stag bettle) and the flowers. Unless they are all normal things, then I'm wrong. Creation of new species always follows mass-extinction anyway, so I'd say that's where the 'sister' thing ties in.

Look at the servants they have too. Akari has Kuon, whatever it is. And Hikari's (Ionious?) is this huge robot-looking thing that can destroy buildings.