To me it just looks like Shio and her brother were abandoned by their mother and left with their brute monster of a father who obviously at the LEAST beat the SNOT out of her brother and did god knows what to her. I think Shio saw her mother die, I think her father got ahold of her and beat her to death, and that's why we saw that scene with her seeing blood and why she's blocking out her mother's face and everything else from her memory. And I think she blames herself. I think perhaps her mother came back to get them as she promised, and met death in the form of the father, and Shio saw it all. In fact, I bet her mother's last words were something akin to not letting go of her hand.
I think Sato, as someone earlier put so eloquently, did indeed "get the bad touch" from her aunt, and enduring it for as long as she seems to have done forced her to withdraw into herself, and as a result feel absolutely nothing. That's why she had no regard for her body or her reputation with the boys, and why she talks about starting to feel things, the first thing being jealousy--she's coming unwound. And she's going to continue to cling as hard to Shio as she can because she sees her as a sort of salvation (just like creepy guy).
I think it will be an interesting turn soon, as I think Shio is going to be faced with facing what she's blocked out, the anger and rage and fear of the person who killed her mother, and then having to work through what those feelings mean if she finds out that Sato, too, is a killer. And I think Sato is going to find out very soon what it feels like to lose something you hold dear, and is probably gonna snap. This story doesn't really lend a lot of flexibility with character development outside the darker things.
