Ie no Naka no S na Koto

This is not just a regular step-sibling relationship. It's deeper than that. *Note* This whole section is going to contain a lot of spoilers

I believe this mangaka is pretty smart in the way they set up the story actually. It's one of those kinds where you have to piece together all the information it gives you by yourself. The conclusion I came up with is pretty dark though, not sure whether some readers are going to like it -----> Shiori and Miyumu are real siblings. The parents broke up when they were children and then later got back together.

Evidence:

- Miyumu's hair color is said to be the same as her fathers' "rare patchy brown".
- In the flashback where the boy said his mother gets her picture every month and that she cries and yells. If the woman was a stranger why would Shiori's dad send the picture? If Shiori was the child of another woman (half siblings) the dad would not send the picture either.
- The parents, according to Shiori, got married straight after they met. Sounds like a patched up marriage.
- At the end Miyumu asks her if she can really say "it's okay" even if they are brother and sister. He knows.
- When he asks her if she remembers the past and she said she cannot, he said it was probably better (she won't piece what he said in the past together with the truth) and that he will forget along with her (forget that they are siblings).

The authors thrown too many hints in there, and I can't think of any other explanation for it. The mothers sadness over losing Shiori might also be why he acts so psycho. He was probably ignored by his mother while she was in depression, and he became jealous and felt un-loved.