I really hope we will get a bit more info on her backstory. Also, at this point i still kinda wonder about the motivation behind her behaviour. Is the whole "lovely older sister" act still only part of the deal she made with him, or did she grew to genuinely feel that way?
Oh, and this chapter gets even more interesting if you go back to chapter 34, page 20-22. She says she couldnt bring back the dead, as if she tried it once. So in the Izanami/Izanagi story, she would take the role of Izanagi, and "the dead she couldnt bring back" (be it as a metapher or in a literal sense) would be Izanami.