I still think the mother might have done something. Not to the level of their father, but Asahi said that he's lost all faith in adults, so I don't know if he feels that because his mother simply lost Shio to a crazy kidnapper. Since this is so psychological, my guess is that the stress of living in fear and having to raise a girl on her own when she probably wasn't self-sufficient financially before may have made her unstable or harsher with Shio than she should have been. It's certainly not unheard of. Like in Aishiteruze Baby.
...Then again, it's also possible that Shio's just projecting her fears of her father onto her last parental figure as well because it was so ingrained in her, so the lashing out really is imagined. Shio's memories are all messed up anyway you put it.